Support FETCH/LIMIT and OFFSET in RelToSqlConverter

Extend RelToSqlConverter to support FETCH/LIMIT and OFFSET.

FETCH is the standard but LIMIT is more common. Therefore we will need to extend the Dialect to know which syntax a particular database prefers. If it accepts both, let's generate standard SQL syntax.

Work has already started in https://github.com/amoghmargoor/incubator-calcite/tree/CALCITE-1003.