Use pending status in external master checks

When the replication status from the external master is checked, the
pending status must be used. This makes sure that the SlaveStatusArray and
the server state are sync.

Also extended the message that was logged when the external master was
lost. By adding the network address there, it makes it easier to see where
the server was replicating from if only the log file is available.

server/modules/monitor/mariadbmon/mariadbserver.hh

@@ -272,6 +272,13 @@ public:
      */
     bool is_slave() const;
 
+    /**
+     * Check if server is a slave of an external server.
+     *
+     * @return True if server is a slave of an external server
+     */
+    bool is_slave_of_ext_master() const;
+
     /**
      * Check if server is running and not in maintenance.
      *