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/transaction.c b/src/libmsc/transaction.c
index 6c12409..190da29 100644
--- a/src/libmsc/transaction.c
+++ b/src/libmsc/transaction.c
@@ -181,6 +181,8 @@
 	return get_value_string_or_null(trans_type_names, type) ? : "trans-type-unknown";
 }
 
+static uint32_t new_call_id = 1;
+
 /*! Allocate a new transaction and add it to network list
  *  \param[in] net Netwokr in which we allocate transaction
  *  \param[in] subscr Subscriber for which we allocate transaction
@@ -212,6 +214,7 @@
 		.log_subsys = subsys,
 		.transaction_id = trans_id,
 		.callref = callref,
+		.call_id = new_call_id++,
 		.net = net,
 		/* empty bearer_cap: make sure the speech_ver array is empty */
 		.bearer_cap = {