Super file copy overwrite should ensure superfile can't be recreated whilst copying

Related to report in : https://track.hpccsystems.com/browse/HPCC-24693

During a super copy, the super file is initially deleted (if it exists), before the subfiles are copied.
When it is complete - it tries to create the new superfile.
But if the superfile by that time already exists, because someone else has created it, it fails with an 'already exists' error.

I think it should clear the super file upfront, not delete it, and lock it (for read) during the copy.
Then add the new subs once it's done.