Convert the message handler to async/await. (#7884)

changelog.d/7884.misc

synapse/handlers/message.py

tests/events/test_snapshot.py

tests/replication/tcp/streams/test_events.py

tests/storage/test_roommember.py

tests/storage/test_state.py

tests/test_utils/event_injection.py

tests/test_visibility.py

tests/unittest.py

tests/utils.py