Replica max_lag in DSN table

when using the option --recursion-method=dsn make it possible to have a max-lag per recursion. this will make it possible to not wait on slowslaves or backup slaves which may possibly fall behind. if no max-lag is given fallback to the default.

The table would then look like this:
CREATE TABLE `dsns` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `parent_id` int(11) DEFAULT NULL,
  `dsn` varchar(255) NOT NULL,
  `max-lag` int(11) NULL,
  PRIMARY KEY (`id`)
);