Weight-based random map rotation

A while ago, I wanted to add a weight-based option for sv_randommaprotation, such that map entries that haven't been played in a while have a higher chance at getting picked, versus map entries that have been played more recently. A Zombie Horde server that I regularly play on has over 80+ maps in the rotation, and I've noticed for a while that although Zandronum picks any map purely at random, it has a tendency to re-pick maps that have just been played, by some luck. Note that this isn't an indication that the feature isn't working properly (it is), but rather that the random picking isn't as desirable as it could be. People on the server regularly complain about this, so I thought I should finally do something about this.

I was originally going to make the weight-based randomization its own option (i.e. sv_randommaprotation would need to be set to 2 to enable this, while sv_randommaprotation = 1 still kept the pure random option). However, I believe that in general, this change will be a benefit to most players, that it shouldn't need to be its own option and might as well work all the time.