oc-calendar: force cache synchronisation for a message before aborting

Sometimes we're trying to get the `objectVersion` of a calendar message,
but this message's entry is not in the cache. The method
`synchroniseCache` won't work in this case, so we try to force the
synchronisation of that particular message in order to get the change
number before aborting.

OpenChange/MAPIStoreGCSFolder.h

@@ -41,6 +41,7 @@
 
 /* synchronisation */
 - (BOOL) synchroniseCache;
+- (BOOL) synchroniseCacheFor: (NSString *) nameInContainer;
 - (void) updateVersionsForMessageWithKey: (NSString *) messageKey
                            withChangeKey: (NSData *) oldChangeKey
                 andPredecessorChangeList: (NSData *) pcl;