Regularize select statement rendering

[plypaul]

• This PR regularizes the handling of different sections in DefaultSqlQueryPlanRenderer.visit_select_statement_node to have a consistent pattern for the different sections.
• Each section calls a separate method to render that section, and each of those methods return the same result object.
• There are no significant logic changes.
• This makes it cleaner to add the rendering for CTEs in a later PR.