counter-based LRU load balancing in master

a counter-based LRU load balancer in master

this PR is for SPARK-979(https://spark-project.atlassian.net/browse/SPARK-979), where @rxin pointed out that the Spark Scheduler is very deterministic.

This implementation is an alternative to https://github.com/apache/incubator-spark/pull/546, where I introduced a LRUMap. This implementation is cleaner while https://github.com/apache/incubator-spark/pull/546 avoid the sorting overhead at every scheduling moment