Remove use of timestamps in failover code

Using timestamps to detect whether MaxScale was active or passive can
cause problems if multiple events happen at the same time. This can be
avoided by separating events into actively observed and passively observed
events. This clarifies the logic by removing the ambiguity of timestamps.

As the monitoring threads are separate from the worker threads, it is
prudent to use atomic operations to modify and read the state of the
MaxScale. This will impose an happens-before relation between MaxScale
being set into passive mode and events being classified as being passively
observed.

include/maxscale/server.h

@@ -136,6 +136,7 @@ typedef struct server
     bool           is_active;      /**< Server is active and has not been "destroyed" */
     bool           proxy_protocol; /**< Send proxy-protocol header to backend when connecting client sessions. */
     int            last_event;     /**< The last event that occurred on this server */
+    bool           active_event;   /**< Event observed when MaxScale was active */
     int64_t        triggered_at;   /**< Time when the last event was triggered */
 #if defined(SS_DEBUG)
     skygw_chk_t    server_chk_tail;