objectcache: optimize merge()/incr() for WinCacheBagOStuff

objectcache: optimize merge()/incr() for WinCacheBagOStuff

Do not hold WinCache locks during merge() callbacks, as that
could be seconds of being blocked (and some callers do not
expect indefinite blocking). Make merge() use the CAS method
which only uses wincache_lock() after callback runs in very
tight getWithToken()/set() cycle.

Make incr() use wincache_lock() since it also is very tight
doGet()/set() cycle with negligable work in between.

Change-Id: I0dde6f62c7e3d4a802470c181570ad4353d0e6ea