Defend against entry node path bias attacks

The defense counts the circuit failure rate for each guard for the past N
circuits. Failure is defined as the ability to complete a first hop, but not
finish completing the circuit all the way to the exit.

If the failure rate exceeds a certain amount, a notice is emitted.

If it exceeds a greater amount, a warn is emitted and the guard is disabled.

These values are governed by consensus parameters which we intend to tune as
we perform experiments and statistical simulations.