Add a safe/sane way to stop a GoRef instance

[mreithub]

Right now GoRef instances are considered permanent (i.e. once you create them, you're stuck with them until the program exits).
That's why spawning an eternal goroutine for each of them isn't that big a deal.

But for the sake of completeness, it should be possible to stop these goroutines in a safe manner (and put the GoRef instance into an inactive state where .Ref() will fail - also in a safe way)