add slave_connections to the list of supported variables for flush status;

With the experience of issue

[MCOL-4556|https://jira.mariadb.org/browse/MCOL-4556]

It was be helpful, if the variable slave_connections could be set to 0.

It is a simple counter, so it should be supportable

[slave_connections|https://mariadb.com/kb/en/replication-and-binary-log-status-variables/#slave_connections]