INT-3112 Fix OOM in SimplePool

The SimplePool maintains an 'allocated' set, for the sole reason
of preventing a "foreign" (non-managed) object being returned.

When a pool item is detected as stale, it is removed from the pool
but remains in the 'allocated' set.

Add a test to verify the allocated size is reduced when a stale
item is popped from the pool.

Add a FileTransferringMessageHandler test (where the problem was
discovered).

Fix a test in TCP to expect a close().

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.