Fixes implementation of `isCurrentQueue`.

This was pretty bad: the check would always return yes, as long as we
would be within a queue created with PARDispatchQueue, as the value
used with the dispatch_queue_get/set_specific was the same for all
instances.
Replacing all uses of `isCurrentQueue` with `isInCurrentQueueStack` in PARStore.

The call is only used for sanity checks within NSAssert. Even with the
previously broken implementation, the assertion was somewhat useful,
surprisingly. With the correct implementation, the assertion could be
too stringent.