Consensus API
Background
This RFC is out-of-scope Iroha 2.0.0.
Initial idea is to define loosely coupled API for `Consensus`. This way Iroha will be able use not only Sumeragi as a consensus algorithm.
It will be better to use some standards here, dor example: https://github.com/hyperledger/sawtooth-rfcs/blob/master/text/0004-consensus-api.md