When doing update-in-place, there is a (small) chance that the thunk
may have been blackholed and another thread might be waiting on it.
We can't therefore just splat it with the value; we have to check
whether it is a blocking queue and awaken any threads that might be
waiting on it if so.

Should fix the scavenge_mutable_list crash reported recently.  If it
does, it will be merged to STABLE.