PERF: multi-column sort_values speedup
Speedup achieved for sorting by multiple columns: lexsort over multiple columns
replaced with mergesort of columns in reverse order than specified in "by"
argument of DataFrame.sort_values()

 - [ ] closes #17111
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master -u -- "*.py" | flake8 --diff``
 - [x] whatsnew entry
