Teach tracker how to value some upload locations more

At present, the tracker picks a random element in the upload location set.  This gives us a uniform distribution of WARCs across upload locations, but sometimes we don't want that because some upload locations have more bandwidth/space than others.

It would be nice to be able to weight certain locations more.  For example, say location A has score 200 and location B and C each had score 50.  In this case we would expect location A to take ~67% of all upload traffic, leaving B and C each with ~16-17% of all upload traffic.

The [weighted-upload-targets](https://github.com/ArchiveTeam/universal-tracker/tree/weighted-upload-targets) branch has some of this work done already.