objectcache: fix WRITE_ALLOW_SEGMENTS in BagOStuff cas() and add() methods

objectcache: fix WRITE_ALLOW_SEGMENTS in BagOStuff cas() and add() methods

Add MediumSpecificBagOStuff::getValueOrSegmentList() helper method.

Also:
* Use $keysMissing variable correctly in CachedBagOStuff::getMulti()
  to avoid extra overhead.
* Optimize mergeViaCas() when the current value matches the new one.

Change-Id: I5c4bd74379bc459216ac0278150ce3aecff3b851