Randomized rarest-first selector behaves like a sequential selector when peers are seeds

[atomashpolskiy]

On behalf of @ckovorodkin

Randomized flavor of bt.torrent.selector.RarestFirstSelector acts much like a sequential rarest-first selector, when most of the peers are seeds, and many or all pieces are equally available. The rarest-first selector would then choose pieces in a sequential fashion:

N = 10
while pieces is not an empty list
    select and remove random piece with index K between 0 and N
    shift all elements with index greater than K to left

E.g. in the simplest case there is one seed, and each piece has availability of 1. It's easy to see that the order of selection will always be skewed to lower indices (because the set of possible next pieces is handicapped by N first elements).