Simplify Object.keys() caching

@arestov, I attempted to simplify your code by eliminating the `list` array as discussed in my comment: https://github.com/feross/bittorrent-swarm/pull/8#issuecomment-62330973

Could I get a code review from you or @fisch0920 before I merge and publish? 
Right here I'm replacing an `array.reduce` with a `for-in` so I don't expect a huge difference, but if it ends up being important, we can change this to something faster.