Also rollback before appending new slices

[forman]

Is your feature request related to a problem? Please describe.

If a running zappend task is interrupted and no rollback can be performed (power cut, sudden VM death) the target dataset will likely be corrupt, i.e., not have its structural integrity preserved.

Describe the solution you'd like

The next time new slices are appended to such an incomplete dataset, zappend should be able to detect its in-progress state and then "heal" it by rolling back the delta with the changes already applied. This requires that the dataset's lock file and the rollback info are still available.