grandpa: Voter persistence and upgrade to finality-grandpa v0.7 (#2139)

* core: grandpa: migrate to grandpa 0.7

* core: grandpa: store current round votes and load them on startup

* core: grandpa: resend old persisted votes for the current round

* core: grandpa: store base and votes for last completed round

* core: grandpa: fix latest grandpa 0.7 changes

* core: grandpa: update to grandpa 0.7.1

* core: grandpa: persist votes for last two completed rounds

* core: grandpa: simplify VoterSetState usage

* core: grandpa: use Environment::update_voter_set_state

* core: grandpa: fix aux_schema test

* core: grandpa: add docs

* core: grandpa: add note about environment assumption

* core: grandpa: don't update voter set state on ignored votes

* core: grandpa: add test for v1 -> v2 aux_schema migration

* core: grandpa: add test for voter vote persistence

* core: grandpa: use grandpa 0.7.1 from crates.io

* core: grandpa: use try_init in test

* core: grandpa: add comment about block_import in test

* core: grandpa: avoid cloning HasVoted

* core: grandpa: add missing docs

* core: grandpa: cleanup up can_propose/prevote/precommit