LRU-Touch objcts for OA_VARY

varnish-cache does not touch objects for OA_VARY, but we need
to keep FCOs in memory which are frequently used during lookup.

Thoughts on why this should not race LRU:

- lru_list is owned by lru_mtx
- object can't go away, because
  - for call from hash, we hold the oh->mtx
  - otherwise, we hold a ref

src/fellow_storage.c

@@ -1029,8 +1029,13 @@ sfemem_getattr(struct worker *wrk, struct objcore *memoc, enum obj_attr attr,
 	assert_memstv(stvfe, stv);
 
 	r = fellow_cache_obj_getattr(stvfe->fc, fco, attr, &len);
+
 	if (slen)
 		*slen = (ssize_t)len;
+
+	if (attr == OA_VARY)
+		(void) fellow_cache_obj_lru_touch(stvfe->fc, fco);
+
 	return (r);
 }