* downloader: fix edgecase where returned index is OOB for downloader

* downloader: documentation
Co-Authored-By: Nholiman <martin@swende.se>

* consensus/clique: fix comment typo

* eth,eth/downloader: fix comment typo

* core: speed up GenerateChain

Use a mock implementation of ChainReader instead of creating
and destroying a BlockChain object for each generated block.

* eth/downloader: speed up tests by generating chain only once

This change reworks the downloader tests so they share a common test
blockchain instead of generating a chain in every test. The tests are
roughly twice as fast now.

* downloader: measure successfull deliveries, not failed

* downloader: fix typos

   __
 <(o )___
  ( ._> /
   `---' 

* eth: made changes to conform better to the golint standards

* eth: fix comment nit

This removes a golint warning: type name will be used as trie.TrieSync by
other packages, and that stutters; consider calling this Sync.

In hexToKeybytes len(hex) is even and (even+1)/2 == even/2, remove the +1.

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

* all: get rid of error when create mdb

* core: clean up variables definition

* all: inline mdb definition

* fix typo

* fix typo

* fix typo

* eth: update higest block we know during the sync if a higher was found

* eth: avoid useless sync in fast sync

* 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.

* 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