cbbackupmgr: list should show more useful information

Background
Current the list subcommand simple lists the files in the backup.

Problem
Users would like to know more about the backups:

• Number of documents and tombstones in each backup
• Number of views, GSI, FTS definitions backed up
• The type of backup: merge, full, incremental
  • For merge it should says the dates of the merge.

We should decide if list should be extended or a new subcommand (info) provides this information.