Capacity scheduler should clean up reservations if it runs tasks on nodes other than where it has made reservations

Currently capacity scheduler makes a reservation on nodes for high memory jobs that cannot currently run at the time. It could happen that in the meantime other tasktrackers become free to run the tasks of this job. Ideally in the next heartbeat from the reserved TTs the reservation should be removed. Otherwise it could unnecessarily block capacity for a while (until the TT has enough slots free to run a task of this job).