time_stats/thread_with_file: promote to lib/ [50/54]

This is Kent Overstreet's series to lift the mean and variance
computation code, the time statistics code, and the thread with file
code to become generic library code.

This has been running on the djcloud for months with no problems.  Enjoy!

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
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