Fix handling of DEFAULT through REST

Pull the DEFAULT logic into PlanGenerator and have all consumers (INSERT, ALTER and REST) use that instead of duplicating logic. 

Required a minor rework of the strategy InsertProcessor used. It's now more similar to OperatorAssembler (ValuesScan -> Project (Defaults) ->Insert) instead of using binds for all the inputs + IFNULL. This not only handles the broken case (e.g. CURRENT_DATE) but also brings the behavior inline with what a NULL literal through SQL does.