Batch up storing state groups when creating new room (#14918)

changelog.d/14918.misc

synapse/events/snapshot.py

synapse/handlers/message.py

synapse/handlers/room.py

synapse/handlers/room_batch.py

synapse/handlers/room_member.py

synapse/storage/databases/state/store.py

tests/handlers/test_message.py

tests/handlers/test_register.py

tests/push/test_bulk_push_rule_evaluator.py

tests/rest/client/test_rooms.py

tests/storage/test_event_chain.py

tests/storage/test_state.py

tests/unittest.py