Updated serialization metrics to do sampling.

The configuration parameter
spark.metrics.serializeTime.sampleProbability controls
the frequency of sampling.

Conflicts:
	core/src/main/scala/org/apache/spark/executor/TaskMetrics.scala
	core/src/main/scala/org/apache/spark/scheduler/JobLogger.scala
	core/src/main/scala/org/apache/spark/scheduler/ShuffleMapTask.scala
	core/src/main/scala/org/apache/spark/serializer/Serializer.scala
	core/src/main/scala/org/apache/spark/storage/BlockObjectWriter.scala