This ticket simplifies the life cycle of live restore metadata over the course of a live restore.

We currently use nbits=-1 to indicate that live restore has completed migrating a file, and then convert that to zero in preparation for the file to be used a source in a future live restore.

Instead, we should now leave all files with nbits=-1 once they have finished migration, even in the CLEAN_UP and COMPLETE stages.

As a result of this change only databases that have been properly backed up can be used as a source directory. Otherwise the future live restore will see nbits=-1 in the source file metadata and assume the unmigrated file has already been migrated.

To protect against this case the ticket will also add a check that the source directory is a proper backup, and if it isn't abort WiredTiger. This will require updates to most, if not all, of our current tests to make use of the proper backup process instead of copy pasting the old WiredTiger directory.