Add container garbage collection.

Work in progress, will add unit tests this evening, assuming this looks ok.
I meant `go util.Forever(...)`

----- Original Message -----
> > @@ -186,6 +186,15 @@ func main() {
> >  		*registryBurst)
> >  
> >  	go func() {
> > +		util.Forever(func() {
> 
> util.Forever is blocking.
> 
> ---
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> https://github.com/GoogleCloudPlatform/kubernetes/pull/2022/files#r19452991