Backoff algorithm

[lmatteis]

An important change is inside the _closest() method, where I also store the v property found in replies. This could potentially increase memory footprint of a node. But I need to store the values somewhere to be able to compare them for the backoff algorithm.

For info on the algorithm check the newly added Expiration section here: http://bittorrent.org/beps/bep_0044.html

Discussion #135

[lmatteis]

Problem with travis failing is that older versions of node don't have Buffer.equals(). Ideas?

[feross]

@lmatteis We already depend on https://www.npmjs.com/package/buffer-equals -- you can just use that.

[lmatteis]

@feross ah sweet. works!

[feross]

@mafintosh Do you have any thoughts on this PR? It's been a while and I think we should merge or close at this point. I'm not familiar with the spec in question, so I'll defer to you

[mafintosh]

Yea, I need to take a closer look. I'll try to make some time over the next week to do so