Merge pull request #3604 from matrix-org/rav/background_process_fixes

changelog.d/3604.feature

synapse/app/homeserver.py

synapse/groups/attestations.py

synapse/replication/tcp/client.py

synapse/replication/tcp/resource.py

synapse/rest/media/v1/media_repository.py

synapse/rest/media/v1/preview_url_resource.py

synapse/storage/devices.py

synapse/storage/event_federation.py

synapse/storage/event_push_actions.py

synapse/storage/transactions.py

tests/replication/slave/storage/_base.py