Implement k-anonymity

[csirianni]

Divide passwords into tables corresponding to the leaked byte(s). That is, each table will be named after the integer value of the leaked byte(s). If we leak only one byte, then the names of tables range from [0, 255].

Limit the # of leaked bytes to 4 (or 3, up to you).

This will change our db initialization inmain.cpp and some unit tests, but don't change the Database class. Try to change as few tests as possible. We still need the key table, but we don't need to passwords table.