Allow maintenance and drain on all servers with --force

MXS-2321 and MXS-3532 changed Maxscale to prevent setting a server into maintenance or drain modes if it's labeled as Master by the monitor. There are cases where this would be the simplest and most convenient method of blocking new connections and/or writes to the Master server. This can be changed so that the --force option in MaxCtrl also allows a server to be set into maintenance mode even if the monitor would not normally allow it.

Original title and description: Rename and expose SERVER_DEMOTING server status

MXS-5555 added the SERVER_DEMOTING status bit which is used to block new transactions. It should be exposed as a settable status and it could be renamed to something like SERVER_READ_ONLY if "demoting" is not good enough.

Unlike the maintenance and drain bits, this must be allowed to be set on all servers as it would be used to prevent writes before a manual shutdown of the server. This way it would be possible to implement a "manual Galera switchover" by setting the Galera node in MaxScale into the new read-only mode and after all transactions have drained, the node could be shut down.