* Problem: Source files contain no license info
Solution: Add comments with SPDX license info to source files
* Python 3 files don't need # -*- coding: utf-8 -*-
* Problem: Validator set not tracked by BigchainDB
Solution: BigchainDB depends on tendermint's RPC API to get the validator set
which is not avaiable during replay so the validators set should be tracked
inside BigchainDB
* Problem: Unclear code and documentation
Solution: Fix decode_validator and docs strings
* Problem: Doc strings missing
Solution: Add doc string for store_validato_set
