Only write monitor journal when it changes

The state of the monitored servers is only persisted if the states of the
servers have changed. This removes the unnecessary disk IO caused by the
writing on the monitor journal.

include/maxscale/monitor.h

@@ -19,6 +19,7 @@
 #include <maxscale/cdefs.h>
 
 #include <mysql.h>
+#include <openssl/sha.h>
 
 #include <maxscale/config.h>
 #include <maxscale/dcb.h>
@@ -206,6 +207,7 @@ struct mxs_monitor
     time_t journal_max_age; /**< Maximum age of journal file */
     uint32_t script_timeout; /**< Timeout in seconds for the monitor scripts */
     bool master_has_failed; /**< Set to true when the latest event is a master_down event */
+    uint8_t journal_hash[SHA_DIGEST_LENGTH]; /**< SHA1 hash of the latest written journal */
     struct mxs_monitor *next;     /**< Next monitor in the linked list */
 };