MXS-3800: Explain lost_slave events

Currently the state change explanations are only added to mariadbmon. They
are less relevant for Galera clusters as they themselves explain why they
change their states but should still be added to make them easier to
analyze.

The event that isn't explained and is most often encountered is the loss
of a Slave status. Most often the loss of a Slave status happens because
either the IO thread or the SQL thread has stopped. Printing the states of
the threads as well as the latest error should hint at what caused the
outage.

The information can be added to the REST API in 2.5 where the monitors can
add extra information to the server JSON.

server/modules/monitor/mariadbmon/mariadbserver.hh

@@ -136,6 +136,7 @@ public:
     GtidList         m_gtid_current_pos;    /* Gtid of latest event. */
     GtidList         m_gtid_binlog_pos;     /* Gtid of latest event written to binlog. */
     SlaveStatusArray m_slave_status;        /* Data returned from SHOW (ALL) SLAVE(S) STATUS */
+    SlaveStatusArray m_old_slave_status;    /* Data from the previous loop */
     NodeData         m_node;                /* Replication topology data */
 
     /* Replication lag of the server. Used during calculation so that the actual SERVER struct is
@@ -169,6 +170,8 @@ public:
      */
     std::string diagnostics() const;
 
+    std::string print_changed_slave_connections();
+
     void update_server(bool time_to_update_disk_space,
                        const mxs::MonitorServer::ConnectionSettings& conn_settings);