Add switchover option which leaves old primary server to maintenance mode

Add an option to MariaDB Monitor switchover which leaves the old primary server as a standalone (not replicating) and put it to maintenance mode. This feature will be useful when performing a rolling server cluster upgrade.

The replicas are upgraded to the new MariaDB Server version first. These servers can continue replicating, as a more recent MariaDB version can replicate from an older version primary. Then, the primary is switched over. As the old primary is still running the older MariaDB version, it should not attempt to replicate from the new primary until it has been upgraded.

Original description:
When performing Rolling Upgrades to Server, I start by upgrading my replicas one at a time. Once all replicas (B&C) are on a new version and the primary (A) is the only server left on an old version, I issue a "SWITCHOVER" Command to promote a newly upgraded replica (B) and the old version primary (A) is automatically made a replica of B.

HOWEVER, this making older version A in a replica of new version B is BAD and can break replication or have all sorts of unsupported bugs.

I need an option in SWITCHOVER to NOT make the current primary A a replica. I need to be able to Switchover to B and put A directly into a standby maintenance mode until upgrades are complete.