Optimize ban lurker and dying objects

When an object is dying, we would rather not have the ban
lurker evalueate it against any existing bans. Now it will,
when a dying object is found, simply remove the oc off the
ban list completely. This is very cheap since we are already
holding the oh and ban mutexes.

bin/varnishd/cache/cache_ban.c

@@ -266,6 +266,8 @@ BAN_DestroyObj(struct objcore *oc)
 {
 
 	CHECK_OBJ_NOTNULL(oc, OBJCORE_MAGIC);
+	if (oc->ban == NULL)
+		return;
 	Lck_Lock(&ban_mtx);
 	CHECK_OBJ_ORNULL(oc->ban, BAN_MAGIC);
 	if (oc->ban != NULL) {

bin/varnishd/cache/cache_ban_lurker.c

@@ -167,8 +167,23 @@ ban_lurker_getfirst(struct vsl_log *vsl, struct ban *bt)
 		oh = oc->objhead;
 		CHECK_OBJ_NOTNULL(oh, OBJHEAD_MAGIC);
 		if (!Lck_Trylock(&oh->mtx)) {
-			if (oc->refcnt == 0 || oc->flags & OC_F_BUSY) {
+			if (oc->flags & OC_F_BUSY) {
 				Lck_Unlock(&oh->mtx);
+			} else if (oc->refcnt == 0 ||
+			    oc->flags & (OC_F_DYING | OC_F_FAILED)) {
+				/*
+				 * We seize the opportunity to remove
+				 * the object completely off the ban
+				 * list, now that we have both the oh
+				 * and ban mutexes.
+				 */
+				noc = VTAILQ_NEXT(oc, ban_list);
+				VTAILQ_REMOVE(&bt->objcore, oc, ban_list);
+				oc->ban = NULL;
+				bt->refcount--;
+				Lck_Unlock(&oh->mtx);
+				oc = noc;
+				continue;
 			} else {
 				/*
 				 * We got the lock, and the oc is not being