Add a command to delete PITR chunks

PITR's extra storage requirements are proportional to the size of the daily oplog. It's possible for that to be larger, even many times larger, than the full size of the database.

Daily backups being kept for a week or two, or weekly backups being kept for a few months, and monthly backups being kept for 12+1 months are common. Keeping the oplog slices all the way back to that time as well would usually be unnecessary

For now you can work around this by finding the old files in pbmPitr subdirectory and deleting them. PBM will deal with missing file ranges if it finds any, this won't cause a fatal error.

But to make it even easier it would be good to add an option to pbm delete-backup to delete just the PITR oplog slices.

I suggest 'pbm delete-backup --pitr-only [--older-than xxxx]'. Another suggestion was to have a 'pbm delete-pitr' subcommand