Setup and cleanup tasks remain in UNASSIGNED state for a long time on tasktrackers with long running high RAM tasks

When a high RAM job's task is scheduled on a tasktracker, the number of slots it occupies will be more than one. If enough such tasks are scheduled, there can come a situation when there are no more slots free on the node but the number of tasks running is less than the number of slots. The jobtracker currently schedules a setup or cleanup task based on how many tasks are running on the system, rather than slots. As a result of this, it can schedule a setup or cleanup task on a tasktracker without any free slots. If the high RAM job's tasks are long running, this will significantly delay the running of the setup or cleanup task, and thus the entire job.