MockTable support for multiple (individual & composite) keys

MockTable currently allows only one key to be specified; the key can be either simple or composite. In real scenarios though a table can have multiple keys.

Consider a table about passengers that have a national id, a passport, and a social security number.  In the passenger example, all three columns can be individual unique keys.

The goal of this ticket is to extend the MockTable API to support multiple keys allowing more flexibility when writing tests.