Merge pull request #3653 from matrix-org/erikj/split_federation

changelog.d/3653.feature

docs/workers.rst

synapse/app/client_reader.py

synapse/app/event_creator.py

synapse/app/federation_reader.py

synapse/app/federation_sender.py

synapse/app/media_repository.py

synapse/federation/federation_server.py

synapse/handlers/federation.py

synapse/replication/http/__init__.py

synapse/replication/http/federation.py

synapse/replication/slave/storage/transactions.py

synapse/server.py

synapse/storage/events.py

synapse/storage/events_worker.py

synapse/storage/room.py