Don't leave quiet EventSource request handlers behind

This is a fix for: https://github.com/rails/rails/issues/10989

Currently in Rails if you have a quiet AC::Live method and the client disconnects the request handler will be stuck and unable to handle any other requests until the server tries to write to the closed socket.

Not a fan of a few things in this pull request but I figured it would be best to get the conversation started.

1. Having to use rack.hijack to do this, but I don't see another way to do it without changing the rack spec, perhaps someone wiser then myself would have some input? I left the other tests in a broken state, am happy to fix if people are okay with this approach.
2. ```thread.raise(ClientDisconnect)``` I am not sure if a thread.kill will leak connections from the AR connection pool? If not perhaps a thread.kill and a on_close handler?

A cleaner solution would be to leave the close detection as a responsibility of the web server however I believe this would require a change in the rack spec as rack would need to fire a on_client_close callback and the rack application (in this case rails) would have to take appropriate action. I thought I would open this PR to get the conversation started.