Don't plan twice for prepared statements

In #147 we added support for prepared statements, but that was done planning the query twice in DuckDB, once during planning to get the the column types of the result and once during execution. While this works, it's clearly suboptimal performance wise, and it's something we should change eventually.

Start by reading issue #147 to understand the existing prepared-statement support, then trace how DuckDB planning is used first to obtain result column types and again during execution. Done means prepared statements no longer plan the query twice while preserving the existing behavior.

Written by the indexing model from the issue text.