This commit adds many comments and removes unused code.
It also removes the EmptyHash function, which had some uses
but was silly.

Fixes #16756

Fix a spelling mistake in comment

trie: cleaner logic, one less func call

* core: use a wrapped `map` and `sync.RWMutex` for `TxPool.all` to remove contention in `TxPool.Get`.

* core: Remove redundant `txLookup.Find` and improve comments on txLookup methods.

trie: support proof generation from the iterator

* vendor: update leveldb package

* ethdb: print warning log if db is performing compaction

* ethdb: update annotation and log

* node: all golint warnings fixed

* node: rm per peter

* node: rm per peter

eth: propagate blocks and transactions async

Fix some typos in comment code and output log

all: collate new transaction events together

travis, appveyor: bump Go release to 1.10.2

This applies spec changes from ethereum/EIPs#1049 and adds support for
pluggable identity schemes.

Some care has been taken to make the "v4" scheme standalone. It uses
public APIs only and could be moved out of package enr at any time.

A couple of minor changes were needed to make identity schemes work:

- The sequence number is now updated in Set instead of when signing.
- Record is now copy-safe, i.e. calling Set on a shallow copy doesn't
  modify the record it was copied from.

travis: try to upgrade android builder to trusty

Go code users can now tag event struct members with `abi:` to specify in what fields the event will be de-serialized.

See PR #16648 for details.

* bmt/*: golint updates for this or self warning

* Update bmt.go

Feed keeps active subscription channels in a slice called 'f.sendCases'.
The Send method tracks the active cases in a local variable 'cases'
whose value is f.sendCases initially. 'cases' shrinks to a shorter
prefix of f.sendCases every time a send succeeds, moving the successful
case out of range of the active case list.

This can be confusing because the two slices share a backing array. Add
more comments to document what is going on. Also add a test for removing
a case that is in 'f.sentCases' but not 'cases'.

* all: get rid of error when create mdb

* core: clean up variables definition

* all: inline mdb definition