Integrate circuit max_cell_queue_size killer with DoS heartbeats

We just merged legacy/trac#25226 (moved) so relays can kill circuits that queue up too many cells.

But we're nervous about picking a threshold that isn't super high, since there's no feedback about whether relays are hitting this event in the wild: the logs are protocol-warn, so nobody, not us not them, will even know if it happens.

teor suggested that we could tie the circuit killing into the DoS heartbeat logs. Something like "and I killed x circuits that had too many cells queued".

This sounds to me like a great idea.