Merge pull request #5732 from matrix-org/erikj/sdnotify

changelog.d/5732.feature

...ystemd-with-workers/system/matrix-synapse-worker@.service

contrib/systemd-with-workers/system/matrix-synapse.service

contrib/systemd/matrix-synapse.service

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/python_dependencies.py