Make the http server handle coroutine-making REST servlets (#5475)

changelog.d/5475.misc

synapse/http/server.py

synapse/rest/consent/consent_resource.py

synapse/rest/key/v2/remote_key_resource.py

synapse/rest/media/v1/config_resource.py

synapse/rest/media/v1/download_resource.py

synapse/rest/media/v1/preview_url_resource.py

synapse/rest/media/v1/thumbnail_resource.py

synapse/rest/media/v1/upload_resource.py

synapse/rest/saml2/response_resource.py

tests/rest/media/v1/test_media_storage.py

tests/unittest.py