MariaDB Monitor should optionally enforce read-only on rejoining slave nodes

On a server that freshly joins a cluster, but is not an active slave, the MariaBD
Monitor will perform "SET GLOBAL read_only=1" and "START SLAVE".

If that server reastarts later the MariaDB Monitor will take no action on it
though, as it detects that slave thread are already running, so that START SLAVE
is not needed.

So read_only will not be set either though, as SET GLOBAL settings do not persist
across server restarts, and such a slave will be running with read_only=0 then,
unless read_only is also enforced via the servers option file.

In an ideal world MaxScale would maybe be able to perform persistent configuration
chagens to monitored servers, but as we don't have that yet (with the exception of
SET PERSISTENT in MySQL 8.0) MaxScale should at least optionally enforce read_only=1
on restarted slaves, e.g. with a "force-slave-read-only=1" monitor option.