model: wake up pending (sleep-set) actions for fences

model: wake up pending (sleep-set) actions for fences

This completes fence backtracking properly by waking up actions when the
appropriate fence interactions occur. This is a little more complicated
than the simple "could_synchronize()" test, since fence synchronization
can involve interaction of more than two actions.

So, pull the complexity of the "should we wake a thread up" computation
into its own function.