remove constantly lib use and switch to enums. (#12624)

changelog.d/12624.misc

mypy.ini

synapse/handlers/events.py

synapse/handlers/federation.py

synapse/handlers/federation_event.py

synapse/handlers/message.py

synapse/state/__init__.py

synapse/storage/databases/main/events_worker.py

synapse/storage/databases/main/search.py

synapse/storage/databases/main/signatures.py

synapse/storage/persist_events.py