Fix up `state_store` naming (#12871)

changelog.d/12871.misc

synapse/handlers/admin.py

synapse/handlers/device.py

synapse/handlers/federation.py

synapse/handlers/federation_event.py

synapse/handlers/initial_sync.py

synapse/handlers/message.py

synapse/handlers/pagination.py

synapse/handlers/room.py

synapse/handlers/room_batch.py

synapse/handlers/search.py

synapse/handlers/sync.py

synapse/push/mailer.py

synapse/state/__init__.py

tests/handlers/test_federation.py