Improve async destroy processing timing

Previous code effectively enforced that all async free ZIOs were
_issued_ within the TXG timeout. But they could take forever to
complete, especially if the required metadata were not in ARC.

This patch introduces periodic waits every 2000 ZIOs, which should
give at least somewhat reasonable TXG timings even for single HDD
pools with empty ARC. And makes them complete within half of the
TXG timeout, since we might still need time to sync DDT and BRT.

While there, change zfs_max_async_dedup_frees semantics to include
also clone and gang blocks, which are similar. Bump the default
value from set long ago to be more forgiving to block cloning
(still not having logs and benefiting from large TXGs), now that
we have better working time limits. The limit now is a possible
amount of dirty data produced by BRT updates.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Signed-off-by: Alexander Motin <alexander.motin@TrueNAS.com>
Closes #18043