WL#3530: Ndb Backup status and progress

Monitoring the status of the backup is quite useful. It should be possible to
tell, using the ndbapi and/or get the information as log events or mgmapi calls :
* if a BACKUP is ongoing in the system
* the progress of the backup, in percent, so it is possible to see (w/o looking
at the FS) how much left there is to be done of the backup.