Merge pull request #3932 from matrix-org/erikj/auto_start_expiring_caches

changelog.d/3932.bugfix

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/federation/federation_client.py

synapse/rest/media/v1/preview_url_resource.py

synapse/state/__init__.py

synapse/util/caches/expiringcache.py

tests/util/test_expiring_cache.py