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/include/osmocom/msc/transaction.h b/include/osmocom/msc/transaction.h
index d2a463d..63070f7 100644
--- a/include/osmocom/msc/transaction.h
+++ b/include/osmocom/msc/transaction.h
@@ -83,6 +83,9 @@
 	/* reference from MNCC or other application */
 	uint32_t callref;
 
+	/* reference that may be used by MGW to identify a call */
+	uint32_t call_id;
+
 	/* if traffic channel receive was requested */
 	int tch_recv;