Update proxy scope to use two condvars

Our proxyscope type currently uses one condition variable to wait/notify from the caller and the callee threads. It's been pointed out that this may not be safe on older versions of glibc, as there used to be a different understanding of whether or not it was acceptable for a condvar notify to notify the thread which called notify (if it later called wait).

See http://austingroupbugs.net/view.php?id=609