br: add total checksum and stats to backupmeta

## Enhancement
right now backupmeta only contains a list of file pointers pointing to the files, each file contains its checksum info and stats. However if backupmeta is somehow corrupted and missed one file, there is no way to detect. We should add a total checksum/stats to the backupmeta as the signature for the entire backup.

Full backup might be easy to do cuz it's a one shot execution and br knows how many files are backed up. Need to think about log backup as it is continuous.