Abstract server active checks

The checks for the server active status are now hidden behind a
macro. This allows for the conditions of the activity to change without
having to alter the code that uses it.

include/maxscale/service.h

@@ -105,7 +105,7 @@ typedef struct server_ref_t
 } SERVER_REF;
 
 /** Macro to check whether a SERVER_REF is active */
-#define SERVER_REF_IS_ACTIVE(ref) (ref->active)
+#define SERVER_REF_IS_ACTIVE(ref) (ref->active && SERVER_IS_ACTIVE(ref->server))
 
 #define SERVICE_MAX_RETRY_INTERVAL 3600 /*< The maximum interval between service start retries */