Use a stream id generator for backfilled ids

synapse/storage/__init__.py

synapse/storage/account_data.py

synapse/storage/events.py

synapse/storage/presence.py

synapse/storage/push_rule.py

synapse/storage/pusher.py

synapse/storage/receipts.py

synapse/storage/state.py

synapse/storage/stream.py

synapse/storage/tags.py

synapse/storage/util/id_generators.py