ASCI: Use a unique call-id for RTP streams

The MGCP protocol features the 'C' (call-id) to identify which
connections belong to the same call. They may be used by MGW for
accounting or management procedures.

So far we sent the MNCC callref as call-id. Instead, add a separate
unique call_id number space. Assign a unique call_id to each
transaction.

Change-Id: I36c5f159fa0b54fb576ff8bd279928b895554793
Related: OS#4854
diff --git a/src/libmsc/msc_ho.c b/src/libmsc/msc_ho.c
index ce3f100..9e4417f 100644
--- a/src/libmsc/msc_ho.c
+++ b/src/libmsc/msc_ho.c
@@ -421,7 +421,7 @@
 		}
 		ran_enc_msg.handover_request.geran.channel_type = &channel_type;
 		ran_enc_msg.handover_request.call_id_present = true;
-		ran_enc_msg.handover_request.call_id = cc_trans->callref;
+		ran_enc_msg.handover_request.call_id = cc_trans->call_id;
 
 		sdp_audio_codecs_to_speech_codec_list(&scl, &cc_trans->cc.local.audio_codecs);
 		if (!scl.len) {