Avoid buffering continuations
Summary:
We used to buffer APPEND continuations, which could result in client
timeouts; The client was able to APPEND very quickly (into the buffer of
guam), but a reply from the server then took a long while until guam
replayed the APPEND from the buffer. That in turn caused the client to
meanwhile time out.
Differential Revision: https://git.kolab.org/D1792