This gets rid of a very old bug where kernel panics when thread is being
woken up and unblocked at the same time on different cores. This
required adding a new lock to SchedulerQueue::Node and adding a cap to
how many threads a threadblocker can simultaneously block. I don't think
I ever block more than five threads on the same ThreadBlocker so this
should be fine.
Bananymous
92e4078287