Add note to backup

[gsmolk]

It should be possible to add a note to a backup copy at the start of backup or via set-backup command. It should be possible view it via show command in JSON format and overwrite or clear via set-backup.
Max size - 1KB ?

[VictorSpirin]

@gsmol issue_146 gitlab branch

[gsmolk]

It would be good to have a test or two, covering this feature, and documentation update.
Also please use pgSetBackupParams structure in do_backup().
And it appears that note cannot be set using set-backup command.

[gsmolk]

Merged