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
abort if attemp to sync an UID in setChangekey fails
Merge pull request #62 from Zentyal/ejhernandez/fix-space-temporal-crash-setChangeKey

Attempt to sync an UID in setChangeKey to avoid crashing