Add snapshots_changed as property

Make dd_snap_cmtime property persistent across mount and unmount
operations by storing in ZAP and restore the value from ZAP on hold
into dd_snap_cmtime instead of updating it.

Expose dd_snap_cmtime as 'snapshots_changed' property that provides a
mechanism to quickly determine whether snapshot list for dataset has
changed without having to mount a dataset or iterate the snapshot list.

It specifies the time at which a snapshot for a dataset was last
created or deleted. This allows us to be more efficient how often we
query snapshots.

Reviewed-by: Ryan Moeller <[email protected]>
Reviewed-by: Alexander Motin <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Umer Saleem <[email protected]>
Closes #13635 

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