Implement full circuit-build-timeout logic

Tor has a complicated set of logic that measures how long circuits take to build, and then adjusts Tor's time-out requirements accordingly. This is important for performance (and possibily security) properties.

• Basic pareto estimator backend
• Set parameters from consensus
• Record measurements from circuits as they are built.
• Record timeouts from circuits as they time out.
• Compute circuit timeouts correctly, given different circuit lengths.
• Actually use timeout estimates to decide when to timeout/abandon a circuit.
• Use correct "doubling" algorithm from path-spec 2.4.5.
• Break histogram-bin ties correctly.
• Make sure all consensus paremeters are implemented.
• Ability to switch (or reconfigure) estimators while running
• Actually use pareto estimator
• Testing.

These are a bit tricky and will want to be separate tickets:

• #142 (closed): Add a function to tell us when we last saw incoming network activity.
• #143 (closed): Persistence for build-time data
• #144 (closed): Get notification about changes in network parameters.
• #145 (closed): Launch testing circuits as needed.