* 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
0a38dbc809