objectcache: avoid using process cache in nested callbacks

objectcache: avoid using process cache in nested callbacks

Because the process cache can be lagged by virtue of blind TTL,
the HOLDOFF_TTL might not be enough to account for it, so avoid
using it when already inside a callback.

Also split of the tests from the MediaWiki test class, so this
does not require DB access anymore.

Change-Id: I743a1233a5efc7f036fad140a9ff8a30b32f8f27