Add a flag to restore to continue from checksum failure

When restoring from a backup created with Rift, we validate check-sums before sending data to the cluster; in an error scenario, we stop restoring if a documents checksum doesn't match. This is the same behavior as all other restore failure cases, however, we should have a flag which forces the restore to continue if a documents checksum is invalid (e.g. in the case of a bit flip).