Merge pull request #220 from rxin/zippart

Memoize preferred locations in ZippedPartitionsBaseRDD

so preferred location computation doesn't lead to exponential explosion.

This was a problem in GraphX where we have a whole chain of RDDs that are ZippedPartitionsRDD's, and the preferred locations were taking eternity to compute.

(cherry picked from commit e36fe55a)
Signed-off-by: Reynold Xin <rxin@apache.org>