keep references of ports and writers and remove them only after a timeout

The GC can collect them any time after the timeout.

Hopefully solves this Issue https://github.com/planthaber/rock-webapp/issues/2

I opted for two things:

1. The timeout parameter to be a GET param (inside the url ../port/write?timeout=30).
2. Instead of adding callbacks for each writer, there is a single callback who cleans all references to writers where the timeout time has passed (like a timeout based garbage collector). Still, there are the other options: callback for each, set callback time to next timeout

What if I want a timeout of less than 5 seconds?  Timer in EM are really really really cheap (it is an event reactor, single threaded ...), so I personally feel that having a global handler is actually worse: you have to poll "often enough", mostly for nothing.