[SPARK-695] In DAGScheduler's getPreferredLocs, track set of visited partitions.

getPreferredLocs traverses a dependency graph of partitions using depth first search.  Given a complex dependency graph, the old implementation may explore a set of paths in the graph that is exponential in the number of nodes.  By maintaining a set of visited nodes the new implementation avoids revisiting nodes, preventing exponential blowup.

Some comment and whitespace cleanups are also included.