Merge worker apps into one. (#6964)

changelog.d/6964.misc

synapse/app/appservice.py

synapse/app/client_reader.py

synapse/app/event_creator.py

synapse/app/federation_reader.py

synapse/app/federation_sender.py

synapse/app/frontend_proxy.py

synapse/app/generic_worker.py

synapse/app/media_repository.py

synapse/app/pusher.py

synapse/app/synchrotron.py

synapse/app/user_dir.py

synapse/replication/slave/storage/events.py

synapse/storage/data_stores/main/pusher.py

tests/app/test_frontend_proxy.py

tests/app/test_openid_listener.py