net.c: Report the node's retransmit timeout as actual timeout

If NSTART > 1, any retransmit timeouts are reported on the difference in
time with an earlier entry in the transmit queue, rather than the actual
delay before the node entry gets retransmitted.

This change reports on the actual delay for a node before it gets retransmitted,
which more accurately records when the node will get retransmitted.

src/net.c

@@ -987,7 +987,8 @@ coap_wait_ack(coap_context_t *context, coap_session_t *session,
 
   coap_log(LOG_DEBUG, "** %s: mid=0x%x: added to retransmit queue (%ums)\n",
     coap_session_str(node->session), node->id,
-    (unsigned)(node->t * 1000 / COAP_TICKS_PER_SECOND));
+    (unsigned)((node->timeout << node->retransmit_cnt) * 1000 /
+                                                         COAP_TICKS_PER_SECOND));
 
 #ifdef COAP_EPOLL_SUPPORT
   coap_update_epoll_timer(context, node->t);