Added support to BACKUP STAGE to maria-backup

This is a port of the code from ES 10.6
See MDEV-5336 for backup stages description.

The following old options are not supported by the new code:
--rsync             ; This is because rsync will not work on tables
                      that are in used.
--no-backup-locks   ; This is disabled as mariadb-backup will always
                      use backup locks for better performance.