manual modification proof policy for mysql.gtid_slave_pos

While MDEV-15393 describes a use case of modifying mysql.gtid_slave_pos via mysqldump
that leads to replication failure, a similar concern exists for abusive changes made by a user.
Preferably direct sql updates to mysql.gtid_slave_pos should not be allowed when replication is under way. And when they are done the gtid/replication slave states must become in sync with them. Specifically should mysql.gtid_slave_pos be manually inserted with a record having a new max of sub_id column, any new record that will be automatically inserted should have its column value greater than the maximum.