Future: API for blocking futures
STM: if a-p-always-delegate fetch states from parked worker on edit-at

If the async-proofs-always-delegate is passed, workers are killed
only when the proof they computed is obsolete.  If one jumps back
to a state that was computed by the worker (and not the master) instead
of (re)computing such state in the master ask the worker to send it
back.