Improve behavior of the `replicant` nodes when the `core` cluster becomes partitioned (for example when a core node leaves the cluster).
Previously, the replicant nodes were unable to rebalance connections to the core nodes, until the core cluster became whole again.
This was indicated by the error messages: `[error] line: 182, mfa: mria_lb:list_core_nodes/1, msg: mria_lb_core_discovery divergent cluster`.

[Mria PR](https://github.com/emqx/mria/pull/123/files)