Every time I looked at the probe code, my mind ended up twisted and confused. A
probe could change the "enabled" state (tracking the temperature) and be removed
at any time (unless the mtx is held), yet the code did not seem to reflect this.

We un-twist my mind by completing the transition to probe states and adding a
chain of two states for the case that a probe is controlled/deleted while its
task is running:

cooling: running probe disabled
deleted: running probe removed (while cooling only)

With this new scheme, we can now have (I think) a clean state diagram (see dot
file):

- a probe begins in the cold state
- from cold, it can either get removed or scheduled via VBP_Control()
- from scheduled, it can go back to cold (via VBP_Control()) or
  be picked up by vbp_thread() to change to running (aka task started)
- once the task finishes, it normally goes back to scheduled,
  but in the meantime it could have changed to cooling or deleted,
  so vbp_task_comple() hadles these cases and either transitions to cold
  or deletes the probe
- if the task can not be scheduled, the same handling happens

We now also remove running probes from the binheap to remove complexity.

Fixes #4108 for good