MXS-1929: Destroy filters separately from services

The previous implementation did not destroy filters that were not used by
services. With the full initialization of filters in filter_alloc, we can
simply traverse the list of created filters and destroy them knowing that
they are all valid.

server/core/internal/filter.h

@@ -63,5 +63,9 @@ bool filter_can_be_destroyed(MXS_FILTER_DEF *filter);
  */
 void filter_destroy(MXS_FILTER_DEF *filter);
 
+/**
+ * Destroy all filters
+ */
+void filter_destroy_instances();
 
 MXS_END_DECLS