Merge branch 'develop' into neilj/disable_hs

changelog.d/3654.feature

synapse/api/constants.py

synapse/api/errors.py

synapse/event_auth.py

synapse/federation/federation_client.py

synapse/federation/federation_server.py

synapse/federation/transport/client.py

synapse/federation/transport/server.py

synapse/handlers/federation.py

synapse/handlers/room.py

synapse/http/matrixfederationclient.py

synapse/replication/slave/storage/events.py

synapse/storage/state.py

tests/utils.py