Fixing vtkThreadedCallbackQueue deadlock

When one is calling `Wait()` on a future, the future will try to un 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 the prior has been popped in the regular
pathe). When such occurenc 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 befor calling
`Invoke()`. From now on, each part of the code about to invoke is
responsible for updating its status.