Refactor statement matching

[sunng87]

Currently we are using string matching to shortcut those set, show statements. However, this approach somehow fragile. Since we are already using sqlparser for rewrite rules, we can get Statement for both simple and extended query. It would be better to rewrite those shortcut code by check the structured Statement.

[sunng87]

@iPeluwa , do you want to work on this refactoring?

[iPeluwa]

@sunng87 Yes please, I would take a dig at it.

[sunng87]

@iPeluwa Let me know if you have any questions. We can start from the statement at https://github.com/datafusion-contrib/datafusion-postgres/blob/master/datafusion-postgres/src/handlers.rs#L463

[iPeluwa]

#164, I would create a follow-up PR that would completely remove the deprecated methods and update all remaining test references. But the core goal is achieved - robust, type-safe statement handling 🎉