Jenkins: triple the size of the build queue

The current throttling of dispatch results in job starvation. Older jobs will fight for a spot in the queue alongside new jobs.

For now, make sure that the full capacity of the Jenkins queue is utilized. Eventually, the scheduler needs to be made aware of more details when deciding whether to enqueue work.

Separately, long queues are hard to cancel. This change will exacerbate that problem. A mechanism is needed to reliably cancel enqueued and dispatched jobs associated with a particular scheduler.