Refactor mining. Remove a few mining goroutines (#338)

* add context

* extract chain events

* run commit in goroutines

* mine only on canonical

* typo

* linters

* fmt

* mark unused methods

* restore stress test

* test single miner

* remove unsafe Trie storage

* remove locks from miner

* restore interrupt

* remove result goroutine

* remove unconfirmedBlocks

* cherry-pick 04a1d475ff1a36ad8f92fec80385df18c52bdc1f

* extract uncles

* one miner succeeded

* restore context cancel

* cleanup

* skip an unstable test

* remove pending state

* use context instead of interrupt func

* calculate sealHash only once

* comment out unstable test

* after merge

* fix after merge

Co-authored-by: ledgerwatch <akhounov@gmail.com>