Add a `Pool.dispose` function

[shonfeder]

If we create a pool using Pool.create, there is not currently a way to dispose of the resources in the pool if we are done with it. But even if we have expensive resources and want to reuse them for some time, we may still want to dispose of the resources within the lifetime of a switch. For this reason, I think it would be helpful to have a function like Pool.dispose that would call dispose on all the resources in the pool, and pop them all from the pool queue.