Update the db driver adaptor to limit results

[dakkad]

For large queues (especially on InnoDB backed storage) building the index and shipping the whole task list slows the task set down to a crawl. This could be modified to use a limit to only pull a selection of tasks. Note that each dispatcher only claims one task at a time so if reading more than the number of dispatchers guaranteed to return a number higher than the number of dispatchers there will be relevant tasks (assuming overdue).

As the cost of reading the list of tasks from the server is a major factor all returned tasks could be run after claiming multiple tasks, e.g. minimising the load on the database (the result set usually contains the whole queue).