transaction: drop special handling for phases and bookmarks generation We are still doing double backups, but now that we have proper location handling this is less of an issue. Dropping this simplifies the code before we add some pending-related logic. This also ensures we actually test the new 'location' mechanism.

transaction: use 'location' instead of 'vfs' objects for file generation The argument is now a location name. The location must be present in the 'vfsmap' provided to the transaction at creation time.

transaction: use 'location' instead of 'vfs' in the addbackup method This unlock the backup of file outside of store (eg: bookmarks).