* 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* Problem: The backends docs are outdated.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* Problem: The abci-marked tests use outdated Mongo conn.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.
