Improvements to WANObjectCache::getWithSetCallback()

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