As per benchmark results below, these changes speed up encoding/decoding of
consensus objects a bit.

    name                             old time/op    new time/op    delta
    EncodeRLP/legacy-header-8           384ns ± 1%     331ns ± 3%  -13.83%  (p=0.000 n=7+8)
    EncodeRLP/london-header-8           411ns ± 1%     359ns ± 2%  -12.53%  (p=0.000 n=8+8)
    EncodeRLP/receipt-for-storage-8     251ns ± 0%     239ns ± 0%   -4.97%  (p=0.000 n=8+8)
    EncodeRLP/receipt-full-8            319ns ± 0%     300ns ± 0%   -5.89%  (p=0.000 n=8+7)
    EncodeRLP/legacy-transaction-8      389ns ± 1%     387ns ± 1%     ~     (p=0.099 n=8+8)
    EncodeRLP/access-transaction-8      607ns ± 0%     581ns ± 0%   -4.26%  (p=0.000 n=8+8)
    EncodeRLP/1559-transaction-8        627ns ± 0%     606ns ± 1%   -3.44%  (p=0.000 n=8+8)
    DecodeRLP/legacy-header-8           831ns ± 1%     813ns ± 1%   -2.20%  (p=0.000 n=8+8)
    DecodeRLP/london-header-8           824ns ± 0%     804ns ± 1%   -2.44%  (p=0.000 n=8+7)

* rlp: pass length to byteArrayBytes

This makes it possible to inline byteArrayBytes. For arrays, the length is known
at encoder construction time, so the call to v.Len() can be avoided.

* rlp: avoid IsNil for pointer encoding

It's actually cheaper to use Elem first, because it performs less checks
on the value. If the pointer was nil, the result of Elem is 'invalid'.

* rlp: minor optimizations for slice/array encoding

For empty slices/arrays, we can avoid storing a list header entry in the
encoder buffer. Also avoid doing the tail check at encoding time because
it is already known at encoder construction time.

Generated by go1.17 fmt ./...

This change improves the performance of encoding/decoding [N]byte.

    name                     old time/op    new time/op    delta
    DecodeByteArrayStruct-8     336ns ± 0%     246ns ± 0%  -26.98%  (p=0.000 n=9+10)
    EncodeByteArrayStruct-8     225ns ± 1%     148ns ± 1%  -34.12%  (p=0.000 n=10+10)

    name                     old alloc/op   new alloc/op   delta
    DecodeByteArrayStruct-8      120B ± 0%       48B ± 0%  -60.00%  (p=0.000 n=10+10)
    EncodeByteArrayStruct-8     0.00B          0.00B          ~     (all equal)

* eth/protocols, prp/tracker: add support for req/rep rtt tracking

* p2p/tracker: sanity cap the number of pending requests

* pap/tracker: linter <3

* p2p/tracker: disable entire tracker if no metrics are enabled

* p2p: remove direct import of cgo-library

* accounts, rpc: more nocgo alternatives

* rpc: move unix path constant into separate file

* accounts/scwallet: address review concerns, remove copy-pasta

* swarm/storage: increase mget timeout in common_test.go

 TestDbStoreCorrect_1k sometimes timed out with -race on Travis.

--- FAIL: TestDbStoreCorrect_1k (24.63s)
    common_test.go:194: testStore failed: timed out after 10s

* swarm: remove unused vars from TestSnapshotSyncWithServer

nodeCount and chunkCount is returned from setupSim and those values
we use.

* swarm: move race/norace helpers from stream to testutil

As we will need to use the flag in other packages, too.

* swarm: refactor TestSwarmNetwork case

Extract long running test cases for better visibility.

* swarm/network: skip TestSyncingViaGlobalSync with -race

As panics on Travis.

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x7e351b]

* swarm: run TestSwarmNetwork with fewer nodes with -race

As otherwise we always get test failure with `network_test.go:374:
context deadline exceeded` even with raised `Timeout`.

* swarm/network: run TestDeliveryFromNodes with fewer nodes with -race

Test on Travis times out with 8 or more nodes if -race flag is present.

* swarm/network: smaller node count for discovery tests with -race

TestDiscoveryPersistenceSimulationSimAdapters failed on Travis with
`-race` flag present. The failure was due to extensive memory usage,
coming from the CGO runtime. Using a smaller node count resolves the
issue.

=== RUN   TestDiscoveryPersistenceSimulationSimAdapter
==7227==ERROR: ThreadSanitizer failed to allocate 0x80000 (524288) bytes of clock allocator (error code: 12)
FATAL: ThreadSanitizer CHECK failed: ./gotsan.cc:6976 "((0 && "unable to mmap")) != (0)" (0x0, 0x0)
FAIL    github.com/ethereum/go-ethereum/swarm/network/simulations/discovery     804.826s

* swarm/network: run TestFileRetrieval with fewer nodes with -race

Otherwise we get a failure due to extensive memory usage, as the CGO
runtime cannot allocate more bytes.

=== RUN   TestFileRetrieval
==7366==ERROR: ThreadSanitizer failed to allocate 0x80000 (524288) bytes of clock allocator (error code: 12)
FATAL: ThreadSanitizer CHECK failed: ./gotsan.cc:6976 "((0 && "unable to mmap")) != (0)" (0x0, 0x0)
FAIL	github.com/ethereum/go-ethereum/swarm/network/stream	155.165s

* swarm/network: run TestRetrieval with fewer nodes with -race

Otherwise we get a failure due to extensive memory usage, as the CGO
runtime cannot allocate more bytes ("ThreadSanitizer failed to
allocate").

* swarm/network: skip flaky TestGetSubscriptionsRPC on Travis w/ -race

Test fails a lot with something like:
 streamer_test.go:1332: Real subscriptions and expected amount don't match; real: 0, expected: 20

* swarm/storage: skip TestDB_SubscribePull* tests on Travis w/ -race

Travis just hangs...

ok  	github.com/ethereum/go-ethereum/swarm/storage/feed/lookup	1.307s
keepalive
keepalive
keepalive

or panics after a while.

Without these tests the race detector job is now stable. Let's
invetigate these tests in a separate issue:
https://github.com/ethersphere/go-ethereum/issues/1245

* swarm/network/stream: newStreamerTester cleanup only if err is nil

* swarm/network/stream: raise newStreamerTester waitForPeers timeout

* swarm/network/stream: fix data races in GetPeerSubscriptions

* swarm/storage: prevent data race on LDBStore.batchesC

https://github.com/ethersphere/go-ethereum/issues/1198#issuecomment-461775049

* swarm/network/stream: fix TestGetSubscriptionsRPC data race

https://github.com/ethersphere/go-ethereum/issues/1198#issuecomment-461768477

* swarm/network/stream: correctly use Simulation.Run callback

https://github.com/ethersphere/go-ethereum/issues/1198#issuecomment-461783804

* swarm/network: protect addrCountC in Kademlia.AddrCountC function

https://github.com/ethersphere/go-ethereum/issues/1198#issuecomment-462273444

* p2p/simulations: fix a deadlock calling getRandomNode with lock

https://github.com/ethersphere/go-ethereum/issues/1198#issuecomment-462317407

* swarm/network/stream: terminate disconnect goruotines in tests

* swarm/network/stream: reduce memory consumption when testing data races

* swarm/network/stream: add watchDisconnections helper function

* swarm/network/stream: add concurrent counter for tests

* swarm/network/stream: rename race/norace test files and use const

* swarm/network/stream: remove watchSim and its panic

* swarm/network/stream: pass context in watchDisconnections

* swarm/network/stream: add concurrent safe bool for watchDisconnections

* swarm/storage: fix LDBStore.batchesC data race by not closing it

The changes allow building WebAssembly applications which use ethclient.Client.

I forgot to update one instance of "go-ethereum" in commit 3f047be5.

All code outside of cmd/ is licensed as LGPL. The headers
now reflect this by calling the whole work "the go-ethereum library".