[PATCH] Simplify SortSupport implementation for macaddr

Hi,

Since Datums are 64-bit values and MAC addresses have only 6 bytes,
the abbreviated key contains the entire MAC address and is
authoritative, as pointed out by John Naylor [1]

This fact eliminates the need for cardinality estimation using
HyperLogLog since macaddr_abbrev_convert() is dirt cheap and not
lossy. There are no reasons to give up on abbreviation.

Potentially we could go even further and pass MAC addresses by value
rather by reference [2]. This would eliminate the need of abbreviation
completely since SortSupport->comparator could just compare two
Datums, as we do for Timestamps. This is a more invasive change though
that deserves more discussion and thus not proposed here.

[1]: https://postgr.es/m/CANWCAZYWdOEnoL_88VpMge1RtRpBz-VRCjdcu-eA4q3U6LvpDw%40mail.gmail.com
[2]: https://postgre.es/m/CAJ7c6TM8up%3DYih8pRLPy4wHwLzHf7w22tQ80-8ZBm__E%3D8_5HA%40mail.gmail.com

-- 
Best regards,
Aleksander Alekseev