Handle multiple replication sources on rejoin

When mariadbmon performs a failover, it currently merges all the replication connections from the old master to the newly promoted master. This poses a problem: if a rejoining server has more than one replication stream configured, the rejoin won't be attempted:

Server 'server2' has multiple slave connections, cannot rejoin.

To fully support an infinite amount of failovers, the rejoining process must clear out all existing replication streams from the server to be rejoined. As this is an irreversible process, it should only be attempted when enforce_simple_topology is enabled.