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/server_utils.hh

@@ -262,6 +262,8 @@ public:
     std::string to_string() const;
     json_t*     to_json() const;
 
+    bool equal(const SlaveStatus& rhs) const;
+
     static slave_io_running_t slave_io_from_string(const std::string& str);
     static std::string        slave_io_to_string(slave_io_running_t slave_io);
     bool                      should_be_copied(std::string* ignore_reason_out) const;