Correctly handle the difference between prev and current state

synapse/api/auth.py

synapse/events/snapshot.py

synapse/handlers/federation.py

synapse/handlers/message.py

synapse/handlers/room_member.py

synapse/push/bulk_push_rule_evaluator.py

synapse/state.py

synapse/storage/roommember.py

synapse/storage/state.py

tests/replication/slave/storage/test_events.py

tests/replication/test_resource.py

tests/test_state.py