Change storage interface to take a slice of peers as input

[shyba]

From a previous discussion on #566, @mrd0ll4r commented:

(...)Thinking about it some more, the most elegant approach would be to pass a (possibly non-empty) slice of bittorrent.Peer to the storage to fill up to numWant, but we can't do that with the current interfaces. (...)

The relevant interface part is AnnouncePeers, from storage.go, with updates to everything implementing it (Redis and memory).