Witness contract random weighting buggy if num backup > 1

[eonwarped]

This is a placeholder tracking item in the event that we decide to increase the number of backup witnesses for P2P.

Right now, the selection for witnesses does a single loop down the rest of the witnesses after the top after choosing one random weight. The problem with this is that if the weight selected is near the end of the list, it will not choose enough witnesses to fill the schedule. The probability of being selected for a particular backup witness slot is also no longer directly proportional to approval weight.