o Minor bugfixes (connection lifespan):
   - Allow more control over how long TLS connections are kept open: unify
     CircuitIdleTimeout and PredictedPortsRelevanceTime into a single option
     called CircuitsAvailableTimeout. Also, allow the consensus to control
     the default values for both this preference, as well as the lifespan
     of relay-to-relay connections. Fixes bug 17592; bugfix on 0.2.5.5-alpha.
   - Increase the intial circuit build timeout testing frequency, to help
     ensure that ReducedConnectionPadding clients finish learning a timeout
     before their orconn would expire. The initial testing rate was set back
     in the days of TAP and before the Tor Browser updater, when we had to be
     much more careful about new clients making lots of circuits. With this
     change, a circuit build time is learned in about 15-20 minutes, instead
     of ~100-120 minutes.