Improve mdl_notifications 3.7->3.9 upgrade script performance

1. Create a fresh Moodle 3.7 instance. 2. Add records: psql> insert into mdl_notifications (useridfrom, useridto, subject, fullmessageformat, timecreated) select i, i, 'This is a subject' , 0 , 1601623098 from generate_series( 1 , 10000000 ) i; psql> insert into mdl_message_popup_notifications (notificationid) select 66000000 from generate_series( 1 , 1000000 ) i; 3. Checkout to the target Moodle version (3.9, 3.10, master). 4. Run upgrade from the GUI. 5.  Confirm , that the `message_popup` progress bar displays the progress. 6. Confirm , that the number of records is going down: psql> select count( 1 ) from mdl_message_popup_notifications; 7. Confirm , that upgrade finished successfully. 8. Repeat 1-3 and run upgrade from the CLI: php admin/cli/upgrade.php 9. Confirm , that upgrade finished successfully.