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

> pass the resolved port to getWriter, it will make the POST fail early if the port does not exist and will allow ...
... getWriter to do the logic if the writer is not there yet (i.e. to create if it does not exist), removing the need for addWriter

That will be mush slower, as getting the port is a CORBA request, this way, there is no further request to the name service once the writer is created (but silently fails, when the port vanishes at runtime)