Add 'zfs wait' command

Add 'zfs wait' command

Add a mechanism to wait for delete queue to drain.

When doing redacted send/recv, many workflows involve deleting files
that contain sensitive data. Because of the way zfs handles file
deletions, snapshots taken quickly after a rm operation can sometimes
still contain the file in question, especially if the file is very
large. This can result in issues for redacted send/recv users who
expect the deleted files to be redacted in the send streams, and not
appear in their clones.

This change duplicates much of the zpool wait related logic into a
zfs wait command, which can be used to wait until the internal
deleteq has been drained.  Additional wait activities may be added
in the future.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Gallagher <john.gallagher@delphix.com>
Signed-off-by: Paul Dagnelie <pcd@delphix.com>
Closes #9707