Allow for replacing target

[forman]

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

While testing the desired configuration for a target data cube, it is often required to replace an existing (test) target before invoking zappend, because zappend requires a non-existing target or a compatible target to which it can append.

An option to replace an existing target hasn't been added so far, because the rollback for deleting large existing targets may be huge and slow.

Describe the solution you'd like

Nevertheless add option to replace an existing target, before the actual append operation.
Emit warning message saying that the replace option is enabled and the deletion is excluded from rollback.

Describe alternatives you've considered

Replace including rollback.