ddt: compare keys 64-bits at a time, trying to match ZAP order

This yields substantial performance improvements when we only write out
some small % of entries at a time, as it will cause entries that will go
into "nearby" ZAP leaf nodes to be grouped closer together in the AVL, and
so touch fewer blocks. Without this, the distribution is an even spread,
so we touch a lot more ZAP leaf nodes for any given number of entries.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Co-authored-by: Allan Jude <allan@klarasystems.com>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Sponsored-by: Klara, Inc.
Sponsored-by: iXsystems, Inc.
Closes #15895