When a master server is set into read-only mode, it can be treated as an
always up-to-date slave. This gives us a somewhat graceful way to prevent
writes to a master.
Usually setting the master into read-only mode is done before a change in
the replication topology to prevent stray writes arriving on the
master. MaxScale should respect the read-only mode and not send any writes
to a server that's in read-only mode.
4cb6d9227e