* 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