Fix slow callbacks in getWithSetCallback() using lockTSE

Fix slow callbacks in getWithSetCallback() using lockTSE

* Keys that take a long time to generate would run into
  the MAX_SNAPSHOT_LAG check and have set() no-op. This
  would be fine except that leaves no key there to figure
  out the time since expiry and therefore whether to use
  the mutex, so it didn't. This now saves the keys but with
  a FLG_STALE bit set, making the next caller that sees it
  perform a regeneration (unless it can't get the mutex).
* Bumped LOCK_TTL so that keys can stay locked much longer.
* This is easy to test via sleep(5) in tagUsageStatistics()
  and two Special:Tags browser tabs.

Bug: T91535
Change-Id: I549e70ace3d2e7da40d3c4346ebacc36024cd522