wireprotov2peer: add TODO about streaming responses

This is a pretty big deficiency in the current implementation.
We don't want to forget about this.

Differential Revision: https://phab.mercurial-scm.org/D4435

mercurial/wireprotov2peer.py

@@ -136,6 +136,9 b' class clienthandler(object):'
         response = self._responses[frame.requestid]
 
         if action == 'responsedata':
+            # This buffers all data until end of stream is received. This
+            # is bad for performance.
+            # TODO make response data streamable
             response.b.write(meta['data'])
 
             if meta['eos']: