* Problem: No good way to check for val set absence.
Solution: Make get_validator_set/get_validators return None/[] when there are no validators yet.
* Problem: Incompatible ABCI chain upgrades.
Solution: Record known chains and sync through InitChain. Triggering the migration and adjusting other ABCI endpoints will follow.
* Problem: `run_upsert_validator_approve` was getting the voting power in a convoluted way
Solution: Changed it to get the voting power from the election outputs
* Problem: `run_upsert_validator_approve` casts votes for *all* voters, not just the user calling the command
Solution: Filter vote txs by the users public key
* Problem: Docs needed a more specific description of how to input the path to the private-key file
Solution: Changed the wording a bit
* 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: RethinkDB, change feed, old mongo, admin interface are not supported any longer.
Solution: Remove unsupported functionality. Bring the MongoDB backend implementation completely to the localmongodb package. Fix the test setup.
* Problem: Nothing depends on multipipes any longer.
Solution: Remove multipipes from setup.py.
* Problem: The how-to-run-tests doc uses --database-backend.
Solution: Do not include the --database-backend option into the documented pytest usage.
* Problem: The backends docs are outdated.
Solution: Document MongoDB as the default and only backend for BigchainDB.
* Problem: The inputs fixtures uses old blocks API.
Solution: Change the inputs fixtures to use the new blocks API.
* Problem: rethinkdb package is not used anymore.
Solution: Remove the rethinkdb dependency from setup.py.
* Problem: The abci-marked tests use outdated Mongo conn.
Solution: Replace MongoDBConnection with LocalMongoDBConnection for them.
