* 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

* cmd, consensus, eth: split ethash related config to it own

* eth, consensus: minor polish

* eth, consenus, console: compress pow testing config field to single one

* consensus, eth: document pow mode

* build: enable unconvert linter

 - fixes #15453
 - update code base for failing cases

* cmd/puppeth: replace syscall.Stdin with os.Stdin.Fd() for unconvert linter

* cmd, consensus, core, miner: instatx clique for --dev

* cmd, consensus, clique: support configurable --dev block times

* cmd, core: allow --dev to use persistent storage too

Tests are now included as a submodule. This should make updating easier
and removes ~60MB of JSON data from the working copy.

State tests are replaced by General State Tests, which run the same test
with multiple fork configurations.

With the new test runner, consensus tests are run as subtests by walking
json files. Many hex issues have been fixed upstream since the last
update and most custom parsing code is replaced by existing JSON hex
types. Tests can now be marked as 'expected failures', ensuring that
fixes for those tests will trigger an update to test configuration. The
new test runner also supports parallel execution and the -short flag.

This PR polishes the EIP 100 difficulty adjustment algorithm
to match the same mechanisms as the Homestead was implemented
to keep the code uniform. It also avoids a few memory allocs
by reusing big1 and big2, pulling it out of the common package
and into ethash.

The commit also fixes chain maker to forward the uncle hash
when creating a simulated chain (it wasn't needed until now
so we just skipped a copy there).

`geth makedag <blocknumber> <path>` was creating DAGs for
`<blocknumber>/<epoch_length> + 1`, hence
it was impossible to create an epoch 0 DAG.

This fixes the calculations in `consensus/ethash/ethash.go` for
`MakeDataset` and `MakeCache`, and applies `gofmt`.

This commit is a preparation for the upcoming metropolis hardfork. It
prepares the state, core and vm packages such that integration with
metropolis becomes less of a hassle.

* Difficulty calculation requires header instead of individual
  parameters
* statedb.StartRecord renamed to statedb.Prepare and added Finalise
  method required by metropolis, which removes unwanted accounts from
  the state (i.e. selfdestruct)
* State keeps record of destructed objects (in addition to dirty
  objects)
* core/vm pre-compiles may now return errors
* core/vm pre-compiles gas check now take the full byte slice as argument
  instead of just the size
* core/vm now keeps several hard-fork instruction tables instead of a
  single instruction table and removes the need for hard-fork checks in
  the instructions
* core/vm contains a empty restruction function which is added in
  preparation of metropolis write-only mode operations
* Adds the bn256 curve
* Adds and sets the metropolis chain config block parameters (2^64-1)

* core: Fix for consensus test gasLimit > 2^63-1 https://github.com/ethereum/tests/blob/develop/BlockchainTests/bcInvalidHeaderTest.json#L238

* core: fix testcase for uncle gasUsage > gasLimit : https://github.com/ethereum/tests/blob/develop/BlockchainTests/EIP150/bcUncleHeaderValiditiy.json#L986

* math/big: rename TTM63m1 -> MaxBig63, + go fmt

* core: documentation

This change removes a convoluted use of sync/atomic from VerifyHeaders.
It also fixes the annoying error about future blocks.