oc: sync old messages as last resort if there are not in cache

This is happening when the lastModSeq is greater than the modseq
from an old message and that message is not in cache.

This is used as last resort before crashing and it is only used
in objectVersion as it is the first place to call on sync. Other
related properties would work.
Merge pull request #59 from Zentyal/ejhernandez/fix-sync-old-msg-as-last-resort

oc: sync old messages as last resort if there are not in cache