Merge branch 'develop' into matthew/preview_urls

scripts/synapse_port_db

synapse/app/homeserver.py

synapse/handlers/_base.py

synapse/handlers/federation.py

synapse/handlers/message.py

synapse/handlers/room_member.py

synapse/replication/resource.py

synapse/replication/slave/__init__.py

synapse/replication/slave/storage/__init__.py

synapse/replication/slave/storage/_base.py

synapse/replication/slave/storage/_slaved_id_tracker.py

synapse/replication/slave/storage/events.py

synapse/state.py

synapse/storage/__init__.py

synapse/storage/_base.py

synapse/storage/engines/__init__.py

synapse/storage/engines/postgres.py

synapse/storage/engines/sqlite3.py

synapse/storage/event_federation.py

synapse/storage/events.py