mysql_replication should not connect to the 'mysql' database

All the actions by the `mysql_replication` plugin can be done by connecting to the `NULL` database. There is no need to connect to the `mysql` db, since there are permissions problems when connecting to the database on a remote host, e.g. when you want to run `getmaster` on a remote host.

This will make it much easier to deploy a master-slave (or master-master) MySQL cluster and everything could be done with the user which will be used for replication. If you create a replication user on both servers with only replication permissions like this:
```GRANT REPLICATION SLAVE, REPLICATION CLIENT ON *.* TO replication@'%' IDENTIFIED BY 'PASSWORD';```

You will be able to connect on a remote host with that same user and run `SHOW MASTER STATUS` which you need if you want to configure replication. The problem is that if you create a user `replication` like this it doesn't have access to the `mysql` database and it doesn't need to have it.