Made WANObjectCache::getCheckKey() automatically init the key

Made WANObjectCache::getCheckKey() automatically init the key

* This still allows If-Modified-Since logic but does not
  need to broadcast initialization values just because a
  key fell out of cache. The value can differ between DCs
  anyway via skew, this just lets them drift more. Actual
  purge events are still broadcasted, which is what matters.
* The User class has now been simplified given this change.
* Added more general comments to getCheckKeyTime().

Change-Id: Ic1f4bbb1947e0d1dd47499c9e9dc86991c30580c