fix async/await consentresource (matrix-org#5585)

Fixes matrix-org#5582

changelog.d/5585.misc

@@ -0,0 +1 @@
+Synapse can now handle RestServlets that return coroutines.

synapse/rest/consent/consent_resource.py

@@ -23,6 +23,8 @@
 import jinja2
 from jinja2 import TemplateNotFound
 
+from twisted.internet import defer
+
 from synapse.api.errors import NotFoundError, StoreError, SynapseError
 from synapse.config import ConfigError
 from synapse.http.server import (
@@ -139,7 +141,7 @@ async def _async_render_GET(self, request):
             else:
                 qualified_user_id = UserID(username, self.hs.hostname).to_string()
 
-            u = await self.store.get_user_by_id(qualified_user_id)
+            u = await defer.maybeDeferred(self.store.get_user_by_id, qualified_user_id)
             if u is None:
                 raise NotFoundError("Unknown user")