This commit changes missioncontrol's store update from per payment to
every second. Updating the missioncontrol store on every payment caused
gradual slowdown when using etcd.
We also completely eliminate the use of the cursor, further reducing
the performance bottleneck.
This commit changes mission control to partially base the estimated
probability for untried connections on historical results obtained in
previous payment attempts. This incentivizes routing nodes to keep all
of their channels in good shape.
