eth: disallow overwrite files via admin.exportChain

* les: wait all task routines before drop the peer

* les: address comments

* les: fix issue

* Change chainId in genesis block to 8888 from 0 

Change chainId in genesis block to 8888 from 0 per Moriteru in https://ethereum.stackexchange.com/a/28082/40230.

* Replace 8888 with “<arbitrary positive integer>”

Per PR review, replace Moriteru’s arbitrary positive integer 8888 with “<arbitrary positive integer>” in chainId field.

cmd, core, eth, les: support --override.istanbul

core/vm: enable istanbul EIPs in the jump table

This change adds support for gzip encoding on HTTP responses.
Gzip encoding is used when the client sets the 'accept-encoding: gzip' header.
Original change by @brianosaurus, with fixes from @SjonHortensius.

Original change by @jpeletier 

This copies cli.printHelp but changes minwidth to 38. Custom flag
code is improved to print the default value using cli.FlagStringer like
all built-in flags do.

build: gomobile automaticall adds the ios tag, don't duplicate

- Move the existing tests from memorydb into a generalized testsuite
that can be run by any ethdb backend implementation.
- Add several more test cases to clarify some non-obvious nuances when
implementing a custom ethdb backend, such as the behaviour of
NewIteratorWithPrefix vs NewIteratorWithStart.
- Add leveldb to the testsuite using in-memory storage for fast
execution.

* core: log chain reorg/split metrics

* core: report 1-block reorgs on the metrics too

crypto/blake2b: fix 386, round 2

crypto/blake2b: fix non-amd64 builds

core/vm, crypto/blake2b: add BLAKE2b compression func at 0x09

The precompile at 0x09 wraps the BLAKE2b F compression function:
https://tools.ietf.org/html/rfc7693#section-3.2

The precompile requires 6 inputs tightly encoded, taking exactly 213
bytes, as explained below.

- `rounds` - the number of rounds - 32-bit unsigned big-endian word
- `h` - the state vector - 8 unsigned 64-bit little-endian words
- `m` - the message block vector - 16 unsigned 64-bit little-endian words
- `t_0, t_1` - offset counters - 2 unsigned 64-bit little-endian words
- `f` - the final block indicator flag - 8-bit word

[4 bytes for rounds][64 bytes for h][128 bytes for m][8 bytes for t_0]
[8 bytes for t_1][1 byte for f]

The boolean `f` parameter is considered as `true` if set to `1`.
The boolean `f` parameter is considered as `false` if set to `0`.
All other values yield an invalid encoding of `f` error.

The precompile should compute the F function as specified in the RFC
(https://tools.ietf.org/html/rfc7693#section-3.2) and return the updated
state vector `h` with unchanged encoding (little-endian).

See EIP-152 for details.

les: handler separation

* blockchain: fix flaw in block import

* core/blockchain: address review concerns

* core/blockchain: go format with 's'

* appveyor: bump to Go 1.12.8

* appveyor: bump to Go 1.12.9

* core/vm, params: implement EIP2200, SSTORE optimizations

* core/vm, params: switch EIP2200 to Wei's version

README: update private network fork config fields

* fourbyte: update signatures, make signatures sorted+not compressed

* fourbyte: disable linter