Unpersist More block manager cleanup.

Implemented a removeRdd method in BlockManager, and use that to
implement RDD.unpersist. Previously, unpersist needs to send B akka
messages, where B = number of blocks. Now unpersist only needs to send W
akka messages, where W = the number of workers.


This commit was based on https://github.com/mesos/spark/pull/633 so please merge that one first.