New-style hint: [NO_]SPLIT_MATERIALIZED

Add a new-style hint to control Split Materialized (aka "Lateral Derived") optimization.

The PR is here: https://github.com/MariaDB/server/pull/4019 .

Hint effect

• NO_SPLIT_MATERIALIZED(X) disables use of Split-Materialized optimization in the context of X (I will elaborate below on what is X).

• SPLIT_MATERIALIZED(X) forces use of Split-Materialized in the context of X unless it is otherwise impossible to do (e.g. because a table is not a materialized derived table, etc).

The hint should be attached to the derived table being split.

It seems a logical and natural location. Consider an example:

SELECT

  /*+ SPLIT_MATERIALIZED(CUST_TOTALS) */

  ...

FROM

  customer

  (SELECT SUM(amount), o_custkey FROM orders GROUP BY o_custkey) as CUST_TOTALS

WHERE

   customer.c_custkey= o_custkey AND

   customer.country='FI';

CUST_TOTALS on its own "does not know" whether Split-materialized would be useful for it.
example: if you need totals for all customers, then probably it won't.
We learn that Split-Materialized is good to use for CUST_TOTALS when we look at the parent query. And realize that we will only require totals for customers from Finland.

SPLIT_MATERIALIZED should force use of splitting

Splitting should be used even when the optimizer would consider it to be not worth it.