add switch_hashtable_insert_destructor so you can insert a pointer into a hash with a custom destructor and use it in spandsp to fix a leak on reloadxml with the tone_descriptor tables and fix a bunch of random tiny leaks etc

src/include/private/switch_hashtable_private.h

@@ -22,6 +22,7 @@ struct entry
     void *k, *v;
     unsigned int h;
 	hashtable_flag_t flags;
+	hashtable_destructor_t destructor;
     struct entry *next;
 };