Implement bwauth cap for latency

Robert, Sebastian and I hashed out an idea for another feedback mechanism for the bw auths based on latency.

Basically, the idea is to create another cap called latency_error similar to how we use circ_error. If a node's latency exceeds some quantile of the population (by being higher than the latencies of say 75% of all nodes), we would then compute and use a pid_error-style error value based on the distance from this 75% quantile setpoint, and use it if it is a more negative number than pid_error and circ_error.

I think the way we want to measure this latency is from CREATE to STREAM FAILED EXITPOLICY for a circuit creation + stream failure for a 1-hop stream exit attempt to localhost. This way we measure both cryptoworker queue latency as well as orconn, circuit, and stream latency.

I think the simplest way to build this is as a separate process from bwauthority.py that simply builds 1-hop circuits and attempts to exit to localhost from them. It would then output a separate, additional measurement file for the network that would be read in by aggregate.py, and used to compute latency_error.