grandpa: observer (#2244)

* grandpa: initial implementation of minimal grandpa worker

* grandpa: extract grandpa observer future to function

* grandpa: add test for observer

* grandpa: start observer if no local key is defined

* grandpa: add minor comments

* grandpa: observer: log invalid commit

* grandpa: observer: persist voter set state on authority change and pause

* grandpa: observer: use commit processing callback

* grandpa: keep run_grandpa to avoid breaking public api

* grandpa: use grandpa::process_commit_validation_result

* grandpa: use finality-grandpa 0.7.2