Refactor insert/materialize node schemas.

Here's a strange case in this sense:
  While you might be tempted to optimize/amortize out most of this string-to-NValue conversion by executing it up-front either per insert plan/executor around plan node loadFromJSON or executor init time or even earlier per TABLE around TableCatalogDelegate/persistenttable construction time, the NOW value can only be evaluated (called) and cached as an NValue at p_execute time. That's probably worth a comment somewhere, like here and/or at the top of this function.