Added proxy weighting #2012

Proposing adding proxy weighting to the current release branch, although I could be convinced otherwise.
> Since you're already using a new instance of the comparator for each sort, you could store the randomized weight for each proxy for a given sort and return those consistently.

I'm not sure I follow. My goal is to have each user sometimes use flashlight proxies according to some probability. Currently the weights aren't randomized but rather the value in the realm of the all weights, which results in the selection. Are you suggesting I store that? If so, that would result in the comparator always choosing either flashlight or standard fallbacks over the other, so you would not get a mix of traffic going to each. Or are you suggesting something else?