WPDB: Force `STRICT_ALL_TABLES` to be enabled as soon as we connect to the MySQL server.

This improves data integrity when inserting and updating rows in the database, particularly when trying to insert emoji into posts stored with character sets that don't support emoji.

See #21212.


git-svn-id: https://develop.svn.wordpress.org/trunk@30400 602fd350-edb4-49c9-b593-d223f7449a82