MINOR: dns/stats: integrate dns counters in stats

Use the new stats module API to integrate the dns counters in the
standard stats. This is done in order to avoid code duplication, keep
the code related to cli out of dns and use the full possibility of the
stats function, allowing to print dns stats in csv or json format.
diff --git a/src/server.c b/src/server.c
index b1656d5..4de4543 100644
--- a/src/server.c
+++ b/src/server.c
@@ -3868,7 +3868,7 @@
 
  save_ip:
 	if (nameserver) {
-		nameserver->counters.update++;
+		nameserver->counters->update++;
 		/* save the first ip we found */
 		chunk_printf(chk, "%s/%s", nameserver->resolvers->id, nameserver->id);
 	}
@@ -3882,7 +3882,7 @@
 
  invalid:
 	if (nameserver) {
-		nameserver->counters.invalid++;
+		nameserver->counters->invalid++;
 		goto update_status;
 	}
 	snr_update_srv_status(s, has_no_ip);