Keeps connections in a Hash to speedup deletion

Using Thin to serve about 10k persistent connections.
Connections occasionally dropped as users leaving the site.
Everything works stable with 1k-2k connections.

But when getting about 10k and a single connection dropped, the app is entirely blocked while `Array.delete` is doing its job.

`Hash.delete` is much faster and adds almost zero latency, so app works stable even with 20k+ connections.

![cp](https://f.cloud.github.com/assets/1230455/160198/94ac6ab6-775b-11e2-8633-aa219a31b089.png)