bagostuff: optimize SqlBagOStuff and fix failing segmentation tests

bagostuff: optimize SqlBagOStuff and fix failing segmentation tests

In SqlBagOStuff:
* Add modifyMulti() helper method to reduce code duplication
* Improve atomicity of add(), cas(), and changeTTL() queries
* Avoid integer serialization and improve atomicity of incr()
* Optimize new BagOStuff::changeTTLMulti() method

In BagOStuff:
* Add changeTTLMulti() method for subclasses to optimize
* Make set() ignore WRITE_ALLOW_SEGMENTS for integers so incr() works
* Strip WRITE_ALLOW_SEGMENTS flag from the setMulti() call in set() to
  avoid triggering bogus sanity check exceptions
* Fix BagOStuffTest::testSetSegmentable failures via the above changes
* Enforce WRITE_ALLOW_SEGMENTS sanity check in setMulti() for all the
  subclasses by using a final wrapper method
* Add WRITE_ALLOW_SEGMENTS sanity check to deleteMulti()

Bug: T113916
Change-Id: I25d1790fa9b0d1837643efccfa94a12043cfbf42