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? 
will be much slower since `for in` iterating much slower usual `for` (and it wrappers such as forEach, map, reduce) iterating over array