Refactored dns_message_t for using attach/detach semantics

This commit will be used as a base for the next code updates in
order to have a better control of dns_message_t objects' lifetime.

(cherry picked from commit 12d6d13100)

lib/samples/sample-request.c

@@ -112,7 +112,7 @@ cleanup:
 	if (qrdataset != NULL) {
 		dns_message_puttemprdataset(message, &qrdataset);
 	}
-	dns_message_destroy(&message);
+	dns_message_detach(&message);
 	return (result);
 }
 
@@ -245,8 +245,8 @@ main(int argc, char *argv[]) {
 	isc_buffer_free(&outputbuf);
 
 	/* Cleanup */
-	dns_message_destroy(&qmessage);
-	dns_message_destroy(&rmessage);
+	dns_message_detach(&qmessage);
+	dns_message_detach(&rmessage);
 	isc_mem_destroy(&mctx);
 	dns_client_destroy(&client);
 	dns_lib_shutdown();