Various clean ups to room stream tokens. (#8423)

changelog.d/8423.misc

synapse/events/__init__.py

synapse/handlers/admin.py

synapse/handlers/device.py

synapse/handlers/initial_sync.py

synapse/handlers/pagination.py

synapse/handlers/room.py

synapse/handlers/sync.py

synapse/notifier.py

synapse/replication/tcp/client.py

synapse/rest/admin/__init__.py

synapse/storage/databases/main/stream.py

synapse/storage/persist_events.py

synapse/types.py

tests/rest/client/v1/test_rooms.py

tests/storage/test_purge.py