Add query-level tunable consistency

This PR provides the ability to supply a `consistencyLevel` query option that specifies the consistency level to use for a CQL3 query.

The `connection.consistencylevel` that is provided when creating the connection is now referred to as the "default consistency level" for queries.

To test, I changed the keyspace used in CQL3 tests to have RF=3. Since the default consistency level of the connections is ONE, all tests continue to succeed. To verify the query-level consistency, I try a simple query overriding the consistency to quorum and ensure it fails.