Alexander Barkov 48ab5a4997 Part2: MDEV-23568 Improve performance of my_{time|date|datetime}_to_str() ...

As an additional improvement, let's store string representations of
the numbers using an array uint16[256] instead of char[512].
This allows to use int2store(), which copies two bytes at a time on x86,
instead of copying the two bytes with digits one-by-one.

This change gives an additional 7% to 26% query time reduce for:
    SELECT BENCHMARK(10*1000*1000,CONCAT(TIME'10:20:30'));
    SELECT BENCHMARK(10*1000*1000,CONCAT(TIME'10:20:30.123456'));
    SELECT BENCHMARK(10*1000*1000,CONCAT(DATE'2001-01-01'));
    SELECT BENCHMARK(10*1000*1000,CONCAT(TIMESTAMP'2001-01-01 10:20:30'));
    SELECT BENCHMARK(10*1000*1000,CONCAT(TIMESTAMP'2001-01-01 10:20:30.123456'));

The total time reduce (part1 + part2) is now between 15% to 38%
for these queries.

2020-09-03 06:03:54 +04:00

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