Added logging for time to open files.

This commit adds logging for both the time HDFS spent opening files
(read using a thread-local variable exposed by HDFS) as well as
logging for the time Spark spends opening shuffle write files.

This commit does not add the open time for the sort based shuffle,
because the shuffle write metrics are currently generally broken for
sort based shuffle.

Conflicts:
	core/src/main/scala/org/apache/spark/executor/TaskMetrics.scala
	core/src/main/scala/org/apache/spark/scheduler/ShuffleMapTask.scala