Filters are now loaded and created at startup

Previously filter instances were created when the first session was made. This
caused filter configuration errors to be noticed only after MaxScale was successfully
started. Now filters are loaded and the instance is created when a service applies its filters.

server/include/filter.h

@@ -107,6 +107,7 @@ typedef struct filter_def {
 
 FILTER_DEF	*filter_alloc(char *, char *);
 void		filter_free(FILTER_DEF *);
+bool        filter_load(FILTER_DEF* filter);
 FILTER_DEF	*filter_find(char *);
 void		filterAddOption(FILTER_DEF *, char *);
 void		filterAddParameter(FILTER_DEF *, char *, char *);