MXS-2652 Do not clear maintenance flag when a server goes down

The set of flags to clear should be well-defined.

include/maxscale/monitor.hh

@@ -118,6 +118,10 @@ public:
     static int64_t get_time_ms();
 
 protected:
+    // When a monitor detects that a server is down, these bits should be cleared.
+    static constexpr uint64_t SERVER_DOWN_CLEAR_BITS {SERVER_RUNNING | SERVER_AUTH_ERROR | SERVER_MASTER
+        | SERVER_SLAVE | SERVER_SLAVE_OF_EXT_MASTER | SERVER_RELAY | SERVER_JOINED | SERVER_NDB};
+
     MonitorInstance(MXS_MONITOR* pMonitor);
 
     /**