Improve async destroy processing timing

[amotin]

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.

How Has This Been Tested?

Deleting many millions of cloned blocks observed TXG commit times reducing to more reasonable values (< 5s), reducing even further (< 1.5s) if sync is explicitly requested by another process.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [x] Performance enhancement (non-breaking change which improves efficiency)
• [ ] Code cleanup (non-breaking change which makes code smaller or more readable)
• [ ] Quality assurance (non-breaking change which makes the code more robust against bugs)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• [x] Documentation (a change to man pages or other documentation)

Checklist:

• [x] My code follows the OpenZFS code style requirements.
• [ ] I have updated the documentation accordingly.
• [ ] I have read the contributing document.
• [ ] I have added tests to cover my changes.
• [ ] I have run the ZFS Test Suite with this change applied.
• [x] All commit messages are properly formatted and contain Signed-off-by.