Polishing

Do not allow returning null items to just release a permit - it
cannot remove the item from allocated. Clients must return the
stale item to the pool so it can be refreshed on the next get.

Only used by the TCP caching CF when returning a connection when
the factory is not running; but should not be allowed.

Also, protect against double release - not currently an issue with
existing users of SimplePool, but should be protected against. Could
cause the permit count to exceed the pool size.

Add inUse Set to the pool so we can detect attempts to release an
item that has already been released.