wireproto: add a _calltwowaystream method on wirepeer

This method will be used by bundle2 pushes. It calls a command, feeds it with a
stream and receives another stream in reply.

Actual implementation for ssh and http will be done in later changesets.

mercurial/wireproto.py

@@ -415,6 +415,13 b' class wirepeer(peer.peerrepository):'
         """
         raise NotImplementedError()
 
+    def _calltwowaystream(self, cmd, fp, **args):
+        """execute <cmd> on server
+
+        The command will send a stream to the server and get a stream in reply.
+        """
+        raise NotImplementedError()
+
     def _abort(self, exception):
         """clearly abort the wire protocol connection and raise the exception
         """