[Reviewer Andy] Fix to issue 87, SAS tracing of CANCEL request/responses

Andy

Can you review please.  I ended up having to fix by changing the code to use a new PJUtils function to send a transaction request, as pjsip_endpt_send_request doesn't give us an opportunity to set the trail ID on the transaction (which is needed both to get the trail ID correct on the request, and to correlate the received response to the right trail).

I've tested live using the live test CANCEL test case and looking at SAS output.  I've also run UTs (and looked at valgrind memory output).

Mike