Fix BackupSystemTable Scans

When calculating which WALs should be included in an incremental backup the backup system does a prefix scan for the last roll log timestamp. This uses the backup root in the prefix (<prefix><backupRoot>.) If you happen have multiple backup roots where one is a root of the other you'll get inaccurate results. 

Since the rowkey is <prefix><backupRoot><null separator><server> let's modify the prefix scan to be <prefix><backupRoot><null separator>.