objectcache: add $flags argument to BagOStuff::delete()

objectcache: add $flags argument to BagOStuff::delete()

This makes it consistent with set() and merge(). Also, one subclass
was already using the field in this manner.

Clean up the code related to WRITE_SYNC in SqlBagOStuff.

Change-Id: I0fb84f4475311889507d3ef98afd4476fb81174f