Table functions: create the result table and return it to the function caller

This task is a part of dj's GSoC project.

Let's denote the query that uses a table function

SELECT ... FROM table_func(args) WHERE ...

as the caller query.

We are not able to execute table functions, yet. Let's make the first step: the code should create a temporary table according to the definition in CREATE FUNCTION ... RETURNS TABLE statement, and pass it to the caller query.

In other words, the caller query should execute as if table_func returned an empty table.

The code inside table_func is irrelevant as far as this task goes. It can be empty or contain some dummy statement.

This task should be done after MDEV-8342 and MDEV-8356 are finished.