Add query classifier plugin.

Basic infrastructure added for being able to load query classifier
plugins using the same mechanism other plugins are loaded with.

server/include/modinfo.h

@@ -50,7 +50,8 @@ typedef enum {
 	MODULE_API_ROUTER,
 	MODULE_API_MONITOR,
 	MODULE_API_FILTER,
-	MODULE_API_AUTHENTICATION
+	MODULE_API_AUTHENTICATION,
+	MODULE_API_QUERY_CLASSIFIER,
 } MODULE_API;
 
 /**