Grow, shrink a pool

This adds the ability to grow, shrink a pool and inspect its mailbox message queue size.

* The minimum pool size is now 1. I have a use case where a pool can be grown, shrunk dynamically via HUP signals. Users may choose to shrink to 1, or even 0 during low traffic periods.

* `mailbox_size` is defined on the ActorProxy, which of course exposes this to all actors. Perhaps this method is also useful for single actors.