Allow diff to be pre-configured for minimal disturbance when enabling/disabling

Right now the diff router requires a fixed topology which it will itself manipulate during the measurement. Being able to use the diff router in a setup where the replication is already up and running is a requirement for cases where changing the topology at runtime is too risky or when the target server is already a normal replicating server.

This scenario is extremely common since the recommended upgrade procedure for MariaDB is to upgrade slaves first and only then upgrade the master. This makes sure that everything works while allowing one to fall back to the old master if things do go wrong.