ddt: lookup and log stats

Adds per-DDT stats counting lookups and where they were serviced from
(either log or backing zap), number of log entries in memory, and flow
rates.

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