Create tool to check integrity of backup files

It would be great to have a tool like cbbackupmgr check to validate the integrity of backup files, and build this as an automatic check after cbbackupmgr backup has been performed.

In a test environment, I took a backup of the Cluster and manually edited one of the .fdb files, effectively corrupting it. Using cbbackupmgr list shows the file as having a size but zero items, and using forest_dbdump produces errors of the form:

[FDB ERR] Crash Detected: Last Block not DBHEADER dd in a database file '/home/backup/bad/2017-11-07T13_50_31.756132596Z/mortimer-9957702c033a6b3bb776c626f9bfff49/data/shard_0.fdb'

[FDB ERR] Crash Detected: Wrong Magic 14259255952413749183 in a database file '/home/backup/bad/2017-11-07T13_50_31.756132596Z/mortimer-9957702c033a6b3bb776c626f9bfff49/data/shard_0.fdb'

Having a tool to easily check whether backups are corrupted seems like a good idea because end users may not know of these tools or what the show, and being able to display a message saying "hey, this backup is corrupted, you should take another" would be useful after the backup/merge/compaction processes.