limit running tasks per job

It should be possible to specify a limit to the number of tasks per job permitted to run simultaneously. If, for example, you have a cluster of 50 nodes, with 100 map task slots and 100 reduce task slots, and the configured limit is 25 simultaneous tasks/job, then four or more jobs will be able to run at a time. This will permit short jobs to pass longer-running jobs. This also avoids some problems we've seen with HOD, where nodes are underutilized in their tail, and it should permit improved input locality.