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ARROW-14778: [C++] Round mean of decimal types after division

With this, the mean and hash_mean aggregate kernels will accumulate sums and perform the final division with decimal values, but the final division will be rounded (half towards infinity) based on the remainder after division instead of rounding towards zero (truncating).

Closes #11754 from lidavidm/arrow-14778

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>