MXS-3158 Preserve character set and collation when altering events

During switchover/failover, server events are altered. The ALTER
EVENT command automatically modifies the event charset and collation
to the values of the connetion running the query. This may cause
the event to become invalid.

Fixed this by changing connection charset and collation to the ones
in the event description just before altering it.

server/modules/monitor/mariadbmon/mariadbserver.hh

@@ -82,6 +82,8 @@ public:
         std::string name;       /**< Event name in <database.name> form */
         std::string definer;    /**< Definer of the event */
         std::string status;     /**< Status of the event */
+        std::string charset;    /**< character_set_client-field */
+        std::string collation;  /**< collation_connection-field */
     };
 
     enum class server_type