[SPARK-9018][MLLIB] add stopwatches

[mengxr]

Add stopwatches for easy instrumentation of MLlib algorithms. This is based on the TimeTracker used in decision trees. The distributed version uses Spark accumulator. @jkbradley

[SparkQA]

Test build #37329 has finished for PR 7415 at commit f981a49.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[jkbradley]

extra space!

[jkbradley]

That's all I see. But also note Sean's comments in the JIRA.

[jkbradley]

triple equals

[mengxr]

Actually @ericl pointed out == and === are equal in this case (Long). Both provide the same error message. I will update it to be consistent.

[jkbradley]

Should we no longer bother with this? Or is it just for Longs (in which case enforcing consistency may be easiest)?

[jkbradley]

LGTM pending tests

[SparkQA]

Test build #37400 has finished for PR 7415 at commit c477745.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[jkbradley]

I'll let you fix that, or merge, as you like

[SparkQA]

Test build #37449 has finished for PR 7415 at commit 40b4347.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[mengxr]

Merged into master.