poll-loop: Remove static variable n_waiters.

poll-loop: Remove static variable n_waiters.

It's always a little risky to track the length of a list by hand, because
it is easy to miss a spot where the length can change.  So it seems like
a small cleanup to just measure the length of the 'waiters' list at the
point where we need to know it.  list_size() is O(n) in the length of the
list, but the function that calls it is already O(n) in that length so it
seems like a fair trade-off.