Add Parser::relates_to_previous_query()

The query classifier needs to know whether a particular statement relates to the previous query or not; e.g. if the function FOUND_ROWS() is used it does.

Currently it calls Parser::get_function_infos() and looks for that function, which is problematic as it implies that the Postgres parser would have to implement get_function_infos() as well. But more importantly, the generic query classifier cannot know what protocol specific functions or statements relate to the previous statement.

This, this functionality needs to be pushed down to the parser.