Convert events worker database to async/await. (#8071)

changelog.d/8071.misc

synapse/event_auth.py

synapse/handlers/federation.py

synapse/handlers/message.py

synapse/handlers/room_member.py

synapse/spam_checker_api/__init__.py

synapse/state/__init__.py

synapse/storage/databases/main/event_federation.py

synapse/storage/databases/main/events_worker.py

synapse/storage/databases/main/stream.py

tests/server_notices/test_resource_limits_server_notices.py

tests/storage/test_appservice.py