#45 Call base->th_notify_fn only as needed

[Nick Mathewson]

Right now, we call th_notify_fn every time we decide we want to wake the event base from another thread. That's needless; we only need to call it if we have not called it already since the last time the event_base woke up.

Adding a flag to fix this, though, would require that we move some locks around and make sure event_base_notify() is only called when holding the th_base_lock, so it's a job for 2.1.x.

[Nick Mathewson]

I believe we fixed this in 4632b78e01a31