Add a field to scanner_info so that we can specify minimum number of distinct bytes necessary in an sbuf to run a scanner

[simsong]

We don't need to run scan_acct unless there are 4 distinct bytes in the sbuf. We don't need to run scan_aes unless there are .. how many bytes in the sbuf? We need to find out how many distinct bytes a scheduled key typically has

• [ ] Update scan_aes so that we can find out how many distinct bytes are associated with a random aes_key and with all zeros but one and the key 0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15
• [ ] Update scan_info so that we can specify to not run a scanner if the amount of distinct bytes isn't present.
• [ ] https://github.com/simsong/be13_api/issues/59 so that we count the number of distinct bytes once and only once.