Attempt to sync an UID in setChangeKey to avoid crashing

This may happen if between sync cache and setChangeKey a modSeq
is making synchroniseCache not retrieve the newly stored message.

This should fix the following crash:

https://tracker.zentyal.org/issues/2673
It cannot happen if the `synchroniseCacheForUID` has do its job. Then, never unless memory corruption and this is the lowest of our problems.