ice transport: avoid deadlock in send callback

deadlock could happen with:
* handleEvents -> pj_lock -> on_data_sent -> iceMutex_
* send() -> iceMutex_ -> pj_ice_strans_sendto2 -> pj_lock

This was fixed by:
* Not setting on_data_sent callback for UDP
* Using a different mutex than iceMutex_ to wait for data send

Change-Id: Ic50698ac9dfe37574f145baa718fff8b74cc99be