switch from list<>.sort() to sort(vector<>)

When I did the original implementation of DocumentSourceSort, there was some issue with using a vector<> of documents. Mathias spotted this in his initial scan of the code. So I went back to see what the problem was. Whatever it was, it's gone, and using a vector<> works now. This is clearly better (because it can internally do something like a qsort()), so I made the change.

Work is already done and tested (was trivial), so I marked it for 2.1.2; will get it through code review.