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ARROW-11568: [C++][Compute] Rewrite mode kernel

Arrow mode kernel performance is bad compared with scipy.stats.mode
(based on numpy.unique). Arrow mode kernel stores value:count pair in
a map, while numpy.unique sorts the input array then count the adjacent
same values. Per my test, the map approach only wins when there are
many duplicated values (length / value_range > 100), looks not very
useful in practice.

This patch rewrites mode kernel to use the sort and count approach for
floating points and integers with wide value range. 2x performance
improvement is observed.

Closes #10009 from cyb70289/11568-mode-optimize

Lead-authored-by: Yibo Cai <yibo.cai@arm.com>
Co-authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>