Merge pull request #3673 from matrix-org/erikj/refactor_state_handler

changelog.d/3673.misc

synapse/api/constants.py

synapse/handlers/federation.py

synapse/handlers/room_member.py

synapse/state/__init__.py

synapse/state/v1.py

synapse/storage/events.py

tests/replication/slave/storage/test_events.py

tests/storage/test_redaction.py

tests/storage/test_roommember.py

tests/test_state.py

tests/test_visibility.py

tests/utils.py