sipcall: use threadpool to destroy temp media transport

IceTransport desctruction shouldn't block on the main thread.
This problem is noticable when calling refuseCall from the UI
thread and results in a blockage lasting MAX_DESTRUCTION_TIMEOUT.
Even if MAX_DESTRUCTION_TIMEOUT is not reached, a polling interval
of 500ms requires this destruction be done async.

Change-Id: Ib6f9c0cff155755de30966875ab76e7288d39405