objectcache: Support key versioning in WANObjectCache

objectcache: Support key versioning in WANObjectCache

* getWithSetCallback() takes a 'version' parameter.
* If the value at a key has a different version, then
  getWithSetCallback() will automatically use a separate
  key. Which value "wins" the main key does not matter.
* Purges are handled by using the main key as a sort of
  check key (with no hold-off). Note that this key is always
  purged on delete().
* Changed stash keys to track the same info as other keys
  both for consistency and because this change needs the
  generation timestamp. Renamed the stash prefix to avoid
  corrupt results with Het Deploy.
* This is useful for things like the User class that use
  versioning and have cross-wiki key access and purges.
  Currently, bumps to version must be deployed to all wikis
  at once, which this aims to avoid.

Change-Id: I26ae62f116e32b48bcf06bc13f8b9e79ae976745