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

Cleaning up these references is not time critical so 5 secs more of less doesn't matter, just like the GC runs itself. 

One could event set this to 30s or more, defeult writer lifetime would be between 30 and 60s

But'ill look into the other options.