Libuv's socket handles not closed when Engine is stopped/restarted

[enricogior]

JX_StopEngine() is supposed to take care of stopping the jxcore engine, but it doesn't close the libuv's socket handles (and presumably all other types of handles) that are still open when the libuv loop has finished.
The result is that a new engine will run code that may fail if trying to bind to an already in-use port.