Added proxy weighting #2012

Proposing adding proxy weighting to the current release branch, although I could be convinced otherwise.
The sort is run every time that LittleProxy requests a fallback order, right?  The mix of traffic happens because each time that we generate a fallback order, it might be sorted differently based on the random value, correct?  I'm just suggesting that you store the random value for each proxy for the entire duration of each sort (i.e. the duration of building a single fallback order) so that the sort algorithm itself doesn't do something unpredictable.