lib-master: Change instances.lock settings to make it less likely to …

…fail

With use_io_notify it's spending less time usleep()ing. Use also a slightly
longer timeout just in case.

If a lot of instances were started at the same time, some of them could have
failed with:
master: Error: file_dotlock_open(.../instances) failed: Resource temporarily unavailable

src/lib-master/master-instance.c

@@ -29,8 +29,9 @@ struct master_instance_list_iter {
 };
 
 static const struct dotlock_settings dotlock_set = {
-	.timeout = 2,
-	.stale_timeout = 60
+	.timeout = 10,
+	.stale_timeout = 60,
+	.use_io_notify = TRUE,
 };
 
 struct master_instance_list *master_instance_list_init(const char *path)