Make WAN cache HOLDOFF_TTL smaller by combining db/snapshot lag

Make WAN cache HOLDOFF_TTL smaller by combining db/snapshot lag

* In the common ~0 second lag case, transactions up to ~7 seconds
  long will have normal set() behavior (instead of just ~5 seconds).
* Like wise with ~0 second transactions tolerating ~7 seconds of
  lag (instead of just ~5).
* The lower hold-off time lets caching resume 3 seconds sooner.

Change-Id: I21e2a0a6915492cec422b6a6af5541937745c15b