Make sure we always respond with the latest sync key

Summary:
Previously if a client e.g. sent a modification resulting in syncKey=4,
but then sent another Sync with syncKey=3, the server would just respond
with syncKey=3 instead of 4, which in Outlooks case resulted in the
client resending the modification that lead to syncKey=4 over and over.

Differential Revision: https://git.kolab.org/D4640