RelOptTableImpl.create always expects QueryableTable type in OptiqSchema...

....TableEntry.

Trying Drill with optiq 0.6-SNAPSHOT and I ran into the class cast problem in ```RelOptTableImpl.create(RelOptSchema schema, RelDataType rowType, final TableEntry tableEntry)``` 

This breaks when the tables are subtypes of Table and not Queryable table. In Drill case all tables implement Table interface and not QueryableTable. This seems to be a regression from issue https://github.com/julianhyde/optiq/issues/222.