Add support for instance local in getPreferredLocations of ZippedPartitionsBaseRDD. Add comments to both ZippedPartitionsBaseRDD and ZippedRDD to better describe the potential problem with the approach

Don't accept generated local directory names that can't be created

fix [SPARK-740] block manage UI throws exception when enabling Spark Streaming

RDD.unpersist: probably the most desired feature of Spark

TimeStampedHashMap.

Also updated unit tests to make sure they are properly testing for
concurrency.

Conflicts:
	core/src/main/scala/spark/SparkContext.scala

a) Add support for hyper local scheduling - specific to a host + port - before trying host local scheduling.

b) Add some fixes to test code to ensure it passes (and fixes some other issues).

c) Fix bug in task scheduling which incorrectly used availableCores instead of all cores on the node.

Pull request to address issues Reynold Xin reported

[Fix SPARK-742] Task Metrics should not employ per-record timing by default

This patch does three things:

1. Makes TimedIterator a trait with two implementations (one a no-op)
2. Makes the default behavior to use the no-op implementation
3. Removes DelegateBlockFetchTracker. This is just cleanup, but it seems like
   the triat doesn't really reduce complexity in any way.

In the future we can add other implementations, e.g. ones which perform sampling.

Add zip partitions interface