This is an attempt to fix the flaky consensus/ethash tests under macOS.

* consensus, miner: stale block supporting

* consensus, miner: refactor seal signature

* cmd, consensus, eth: add miner noverify flag

* cmd, consensus, miner: polish

* miner: commit state which is relative with sealing result

* consensus, core, miner, mobile: introduce sealHash interface

* miner: evict pending task with threshold

* miner: go fmt

* consensus/ethash: start remote ggoroutine to handle remote mining

* consensus/ethash: expose remote miner api

* consensus/ethash: expose submitHashrate api

* miner, ethash: push empty block to sealer without waiting execution

* consensus, internal: add getHashrate API for ethash

* consensus: add three method for consensus interface

* miner: expose consensus engine running status to miner

* eth, miner: specify etherbase when miner created

* miner: commit new work when consensus engine is started

* consensus, miner: fix some logics

* all: delete useless interfaces

* consensus: polish a bit

* keystore, ethash, eth, miner, rpc, whisperv6: tech debt with now defunct set.

* whisperv5: swap out gopkg.in/fatih/set.v0 with supported set

correct the algorithm in the comments for fakeBlockNumber， from "min" to "max".

"proot-of-work" to "proof-of-work"

Use Read instead of Sum to avoid internal allocations and
copying the state.

name                      old time/op  new time/op  delta
CacheGeneration-8          764ms ± 1%   579ms ± 1%  -24.22%  (p=0.000 n=20+17)
SmallDatasetGeneration-8  75.2ms ±12%  60.6ms ±10%  -19.37%  (p=0.000 n=20+20)
HashimotoLight-8          1.58ms ±11%  1.55ms ± 8%     ~     (p=0.322 n=20+19)
HashimotoFullSmall-8      4.90µs ± 1%  4.88µs ± 1%   -0.31%  (p=0.013 n=19+18)

* all: get rid of error when create mdb

* core: clean up variables definition

* all: inline mdb definition

* go-metrics: fork library and introduce ResettingTimer and InfluxDB reporter.

* vendor: change nonsense/go-metrics to ethersphere/go-metrics

* go-metrics: add tests. move ResettingTimer logic from reporter to type.

* all, metrics: pull in metrics package in go-ethereum

* metrics/test: make sure metrics are enabled for tests

* metrics: apply gosimple rules

* metrics/exp, internal/debug: init expvar endpoint when starting pprof server

* internal/debug: tiny comment formatting fix

This commit reduces database I/O by not writing every state trie to disk.

* consensus/ethash: add maxEpoch constant

* consensus/ethash: improve cache/dataset handling

There are two fixes in this commit:

Unmap the memory through a finalizer like the libethash wrapper did. The
release logic was incorrect and freed the memory while it was being
used, leading to crashes like in #14495 or #14943.

Track caches and datasets using simplelru instead of reinventing LRU
logic. This should make it easier to see whether it's correct.

* consensus/ethash: restore 'future item' logic in lru

* consensus/ethash: use mmap even in test mode

This makes it possible to shorten the time taken for TestCacheFileEvict.

* consensus/ethash: shuffle func calc*Size comments around

* consensus/ethash: ensure future cache/dataset is in the lru cache

* consensus/ethash: add issue link to the new test

* consensus/ethash: fix vet

* consensus/ethash: fix test

* consensus: tiny issue + nitpick fixes

* accounts, consensus, core, eth: make chain maker consensus agnostic

* consensus, core: move CalcDifficulty to Engine interface

* consensus: add docs for calcDifficulty function

* consensus, core: minor comment fixups