seed / destroy race condition prevent destruction

[mvayngrib]

to repro:

client.seed()
client.destroy()

seed() calls self.add asynchronously and ends up adding to self.torrents after destroy empties it

I suggest something like bittorrent-dht has - self._destroyed with checks, and can implement it if you like. However, this will likely force changes to the ontorrent callback signature. Currently it's not a traditional node callback - ontorrent(torrent), but it will likely need to become one, a la:

if (self._destroyed) ontorrent && ontorrent(new Error('client is destroyed'))

[feross]

Good catch. I'll fix this up.

We don't actually need to make the ontorrent take an error callback. Calling destroy just means that the 'torrent' event will never get emitted. That's how it works with sockets and the 'listen' event in node.js. For example:

var net = require('net')
var socket = net.connect(4000, function () {
  console.log('listen')
})
socket.destroy()

will not log listen.

[feross]

Fixed this issue, added tests, and released as 0.27.0.