SPARK-1284: Fix improper use of SimpleDateFormat

`SimpleDateFormat` is not thread-safe. Some places use the same SimpleDateFormat object without safeguard in the multiple threads. It will cause that the Web UI displays improper date.

This PR creates a new `SimpleDateFormat` every time when it's necessary. Another solution is using `ThreadLocal` to store a `SimpleDateFormat` in each thread. If this PR impacts the performance, I can change to the latter one.

Author: zsxwing <zsxwing@gmail.com>

Closes #179 from zsxwing/SPARK-1278 and squashes the following commits:

21fabd3 [zsxwing] SPARK-1278: Fix improper use of SimpleDateFormat

Conflicts:

	core/src/main/scala/org/apache/spark/scheduler/JobLogger.scala
	core/src/main/scala/org/apache/spark/ui/jobs/JobProgressUI.scala
	core/src/main/scala/org/apache/spark/ui/jobs/StagePage.scala
	core/src/main/scala/org/apache/spark/ui/jobs/StageTable.scala
	core/src/main/scala/org/apache/spark/util/FileLogger.scala