atproto_firehose.subscribe: dark read memcache to compare against DIDs in memory

for #1341

atproto_firehose.py

@@ -171,6 +171,9 @@ def subscribe():
         if not repo:
             logger.warning(f'Payload missing repo! {payload}')
             continue
+        # TODO: remove
+        elif repo in atproto_dids and not ATProto.get_by_id(repo, use_datastore=False):
+            report_error('ATProto user missing from memcache', user=repo)
 
         seq = payload.get('seq')
         if not seq: