cbbackupmgr should Checksum Backup Files

We've seen in testing that if a backup file on disk gets corrupted when the Enterprise Backup tool, cbbackupmgr tries to perform actions such as a merge it can cause a segfault and crash.  This is difficult to debug and as such we should checksum the files on disk to avoid such an issue.

There's various ways that a backup file can get corrupted, here's a page with some examples https://www.sqlite.org/howtocorrupt.html