Persist CreateRoom events to DB in a batch (#13800)

changelog.d/13800.misc

synapse/handlers/message.py

synapse/handlers/room.py

synapse/handlers/room_batch.py

synapse/handlers/room_member.py

synapse/replication/http/__init__.py

synapse/replication/http/send_event.py

synapse/replication/http/send_events.py

tests/handlers/test_message.py

tests/handlers/test_register.py

tests/storage/test_event_chain.py

tests/unittest.py