Inspect circuit queue during padding decisions

We need to inspect the circuit queue or the channel outbuf in some way during padding decisions. The problem is that if a guard stops reading on a channel, and padding keeps getting scheduled by the middle, it will overflow the circuit queue and/or outbuf for the channel and eventually oom.

Ideally, we would make our padding decision based on the EWMA values for the circuit rather than just checking if there were queued cells in the outbuf, but at minimum we need some kind of throttling so we don't keep adding cells to an circuit queue or outbuf above a certain length.

We may also want to use the circuit queue activity to update our last packet sent timers..