This commit adds four functions: GetSeeders, GetLeechers,
NumSeeders, and NumLeechers to the store/peer_store API. The
first two functions are for getting a list of all the
seeders/leechers by an info hash, the latter two are helper
functions which use the Get functions, combine the ipv4, and
ipv4, and then len() them. It also adds some minimal tests for
memory/peer_store.
