Convert storage layer to async/await. (#7963)

changelog.d/7963.misc

synapse/storage/persist_events.py

synapse/storage/purge_events.py

synapse/storage/state.py

tests/storage/test_purge.py

tests/storage/test_room.py

tests/storage/test_state.py

tests/test_visibility.py

tests/utils.py

tox.ini