[SPARK-2950] Add gc time and shuffle write time to JobLogger

The JobLogger is very useful for performing offline performance profiling of Spark jobs. GC Time and Shuffle Write time are available in TaskMetrics but are currently missed from the JobLogger output. This patch adds these two fields.

~~Since this is a small change, I didn't create a JIRA. Let me know if I should do that.~~

cc kayousterhout

Author: Shivaram Venkataraman <shivaram@cs.berkeley.edu>

Closes #1869 from shivaram/job-logger and squashes the following commits:

1b709fc [Shivaram Venkataraman] Add a space before GC_TIME
c418105 [Shivaram Venkataraman] Add gc time and shuffle write time to JobLogger