Fixing vtkThreadedCallbackQueue deadlock

When one is calling Wait() on a future, the future will try to run its priors if they are enqueued (not on hold) using a constant-time lookup through the InvokerIndex member. There was a race condition with the regular path for running tasks. The queue could try to run the prior while the user does it as well. When this happens, the computed index in the queue is negative (because it has been popped by the regular path). When such occurence happens, we abandon trying to run the prior.

Setting the invoker's status to RUNNING was done in Invoke(). It turns out that there are cases when we want to set it before calling Invoke(). From now on, each part of the code about to invoke is responsible for updating this status.