Deduplicate some code in synapse.app (#4567)

changelog.d/4567.misc

synapse/app/_base.py

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/homeserver.py

synapse/app/media_repository.py

synapse/app/pusher.py

synapse/app/synchrotron.py

synapse/app/user_dir.py

synapse/config/logger.py