Fix for waiting connections handling for cancel requests

When the connection pool is full, cancel requests cannot get through (that is normal), but then when unused connections close and pool slots are available, those are not used for waiting cancel requests.

The fix is to launch new connections when cancel requests are queued and pool slots are available.

closes #542
see also #245

src/janitor.c

@@ -174,6 +174,12 @@ static void per_loop_activate(PgPool *pool)
 	PgSocket *client;
 	int sv_tested, sv_used;
 
+	/* if there is a cancel request waiting, open a new connection */
+	if (!statlist_empty(&pool->cancel_req_list)) {
+		launch_new_connection(pool);
+		return;
+	}
+
 	/* see if any server have been freed */
 	sv_tested = statlist_count(&pool->tested_server_list);
 	sv_used = statlist_count(&pool->used_server_list);