Adding support for WITH_VALIDATE to ALTER TABLE EXCHANGE PARTITION

Add support for WITH VALIDATE | WITHOUT VALIDATE to
ALTER TABLE ... EXCHANGE PARTITION ... WITH TABLE
MySQL has had this since 5.7.5 and it's important when adding big tables to a partitioned table to avoid the expensive row-by-row check.

When WITHOUT VALIDATION is specified, there is no check that all rows in the added table fulfills the partition restrictions. If there is inconsistencies, it will result in rows later not being found in SELECT, UPDATE or DELETE.

WITH VALIDATION should be the default behavior.