Add local group server support

synapse/federation/transport/client.py

synapse/federation/transport/server.py

synapse/groups/groups_server.py

synapse/handlers/groups_local.py

synapse/rest/__init__.py

synapse/rest/client/v2_alpha/groups.py

synapse/server.py

synapse/storage/__init__.py

synapse/storage/group_server.py

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