Merge pull request #2378 from matrix-org/erikj/group_sync_support

synapse/app/synchrotron.py

synapse/handlers/groups_local.py

synapse/handlers/sync.py

synapse/replication/slave/storage/groups.py

synapse/replication/tcp/streams.py

synapse/rest/client/v2_alpha/sync.py

synapse/storage/__init__.py

synapse/storage/group_server.py

synapse/storage/schema/delta/43/group_server.sql

synapse/streams/events.py

synapse/types.py

tests/rest/client/v1/test_rooms.py