hashmap: expose helper for releasing memory pools independently of valgrind

Let's clean this up and export this always, so that we can later call
when we are under memory pressure.

src/basic/hashmap.h

@@ -443,3 +443,5 @@ DEFINE_TRIVIAL_CLEANUP_FUNC(OrderedHashmap*, ordered_hashmap_free_free_free);
 DEFINE_TRIVIAL_CLEANUP_FUNC(IteratedCache*, iterated_cache_free);
 
 #define _cleanup_iterated_cache_free_ _cleanup_(iterated_cache_freep)
+
+void hashmap_cleanup_pools(void);