Merge pull request #6454 from matrix-org/erikj/clean_base_Store

changelog.d/6454.misc

synapse/replication/slave/storage/_base.py

synapse/storage/_base.py

synapse/storage/data_stores/main/__init__.py

synapse/storage/data_stores/main/cache.py

synapse/storage/data_stores/main/client_ips.py

synapse/storage/data_stores/main/devices.py

synapse/storage/data_stores/main/events_worker.py

synapse/storage/data_stores/main/registration.py