Merge branch 'akappend-streaming'

X-AKAPPEND handler now supports payload streaming, which should speed up
appending new items into Akonadi.

If clients supports streaming with X-AKAPPEND, it has to announce the
capability to server via AKAPPENDSTREAMING capability.

This effectively obsoletes the APPEND command, which supports streaming for
a long time now, but can only append items which just one payload part
(i.e. contacts and events, but usually not emails), so it was used only
for single-part items for speed. With X-AKAPPEND supporting streaming,
it can be used instead of APPEND without performance penalties.