Rename database classes to make some sense (#8033)

changelog.d/8033.misc

docs/user_directory.md

scripts-dev/update_database

scripts/synapse_port_db

synapse/app/_base.py

synapse/app/generic_worker.py

synapse/app/homeserver.py

synapse/config/database.py

synapse/events/snapshot.py

synapse/handlers/federation.py

synapse/handlers/message.py

synapse/handlers/presence.py

synapse/module_api/__init__.py

synapse/replication/slave/storage/_base.py

synapse/replication/slave/storage/account_data.py

synapse/replication/slave/storage/appservice.py

synapse/replication/slave/storage/client_ips.py

synapse/replication/slave/storage/deviceinbox.py

synapse/replication/slave/storage/devices.py

synapse/replication/slave/storage/directory.py