Add option for pjsua callback on_stream_created to destroy application's supplied media port

[ming]

In pjsua2, when Call object is about to be destroyed, it will call:

pjsua_call_set_user_data(id, NULL);
hangup(prm);

which will prevent any further callbacks to be invoked, meaning that application creating pjmedia_port in onStreamCreated() may not get the notification to destroy it in onStreamDestroyed().

This ticket is to add a new callback on_stream_created2() with the option to allow pjsua to manage the pjmedia port destruction instead.

Thanks to Kal Conley (b17c0de) for the original patch.

[ming]

In 5676:

Fixed #2052: Add option for pjsua callback on_stream_created to destroy application's supplied media port