Improvements to WANObjectCache::getWithSetCallback()
* If lockTSE is set, make sure it applies during during
the entire duration of a key being tombstoned. This lets
regenerations happen during that whole time, which lowers
the chance of seeing stale data if the DBs are lagged.
* If lockTSE is not set, do not apply tempTTL (for tomstoned keys).
If traffic is high, a stale value would usually be "stashed" and
used for 5 seconds. If the lag was only 1 second, then this
is suboptimal.
* Determine tempTTL from lockTSE as they no longer make sense
being separate. This makes things easier to understand and
also makes the lockTSE value account for the last regeneration
time (via stash key TTL). Since LOCK_TSE << HOLDOFF_TTL, this
helps avoid stale reads without adding stampede risk.
Change-Id: I3b01f0ec67935a238b30e02e42004fd3b2dcfb9d