grandpa: pluggable voting rules (#3673)

* grandpa: support pluggable custom voting rules

* grandpa: add docs to grandpa voting rule

* grandpa: make voting rule mandatory

* grandpa: add test for voting rule

* node: add GRANDPA voting rule

* grandpa: pass backend to VotingRule

* core: fix docs in SelectChain::finality_target

* grandpa: implement 3/4 of unfinalized chain restriction as voting rule

* grandpa: rename AlwaysBehindBestBlock voting rule

* grandpa: fix tests

* grandpa: remove useless test

* grandpa: extend environemnt voting rule test

* grandpa: add proofs to unreachable statements

* grandpa: fix typo

* grandpa: fix docs