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/server.h

@@ -140,6 +140,11 @@ typedef struct server
 #define SERVER_STALE_SLAVE       0x2000  /**<< Slave status is possible even without a master */
 #define SERVER_RELAY_MASTER      0x4000  /**<< Server is a relay master */
 
+/**
+ * Is the server valid and active
+ */
+#define SERVER_IS_ACTIVE(server) (server->is_active)
+
 /**
  * Is the server running - the macro returns true if the server is marked as running
  * regardless of it's state as a master or slave