Code review comment for lp:~kdub/mir/server-sent-buffer

> Isn't that the purpose of the done callback? Why is there the additional
> submitting condition variable, and the wait on that?

For now (in the intermediate form), before mcl::BufferStream::next_buffer() returns, we have to wait for two things to happen... for submit buffer to finish and for the new buffer to come back over ipc. If we had a buffer, then we could just process things in the done callback. If the event-buffer arrives after, however, we can wait in the callback (blocking RPC processing). It seemed easiest to just wait for submit buffer to be done like in this MP.