priority-based replica selection in mariadbmon failover

When multiple replicas are equally up to date and suitable for promotion, there is no configurable mechanism for mariadbmon to select the best promotion replica to express a preference among them.

This enhancement proposes extending mariadbmon to support priority-based replica selection by utilizing the existing priority server attribute, similar to how it is used by galeramon.

This would provide greater flexibility for asynchronous replication deployments without altering the current failover logic for determining eligible promotion candidates.

The idea is to extend MaxScale to include this functionality, following the same logic of negative value means DO NOT USE the server, and lowest value has a higher preference. This provides the user with a more flexible approach in selecting up to date replicas. In practice MaxScale will most likely not trigger a failover in sub-second timescales so by then multiple replicas may be in the same state replicating from the now dead master. Optionally adding this final check if use_priority=true and setting priorities to individual hosts would be a useful improvement.