[SPARK-3570] Include time to open files in shuffle write time.

Opening shuffle files can be very significant when the disk is
contended, especially when using ext3. While writing data to
a file can avoid hitting disk (and instead hit the buffer
cache), opening a file always involves writing some metadata
about the file to disk, so the open time can be a very significant
portion of the shuffle write time. In one job I ran recently, the time to
write shuffle data to the file was only 4ms for each task, but
the time to open the file was about 100x as long (~400ms).

When we add metrics about spilled data (#2504), we should ensure
that the file open time is also included there.

Author: Kay Ousterhout <kayousterhout@gmail.com>

Closes #4550 from kayousterhout/SPARK-3570 and squashes the following commits:

ea3a4ae [Kay Ousterhout] Added comment about excluded open time
fdc5185 [Kay Ousterhout] Improved comment
42b7e43 [Kay Ousterhout] Fixed parens for nanotime
2423555 [Kay Ousterhout] [SPARK-3570] Include time to open files in shuffle write time.