Count a dead letter by when it was kept, so one kept late still counts (issue 440)

The give-up time is not newer for every letter kept: a notice that could
not be kept is offered again a minute later, so a later give-up can be kept
first and the one after it read as not fresh. The stored time rises with the
stream's sequence. A consumer whose letters vanish between the two reads is
left out of the look instead of counted as a look, and the skip of a
token-less max-deliveries advisory now has a test of its own.

cmd/mesh-controller/watchdogs.go

@@ -80,8 +80,8 @@ type signalFacts struct {
 	// deadLetters are how many messages DEAD_LETTERS holds per consumer, by `<stream>.<consumer>`
 	// (novox/hq issue 330): each consumer's max-deliveries condition is open while it holds any.
 	deadLetters map[string]int
-	// deadLettersNewest is when each of those consumers last gave up on a message DEAD_LETTERS still
-	// holds: the condition counts the messages given up on, not the looks (novox/hq issue 440).
+	// deadLettersNewest is when DEAD_LETTERS kept the newest message it holds for each of those
+	// consumers: the condition counts the messages given up on, not the looks (novox/hq issue 440).
 	deadLettersNewest map[string]time.Time
 	advisoriesErr     error
 
@@ -355,6 +355,13 @@ func (w *watchdogs) gather(ctx context.Context) *signalFacts {
 	}
 	if f.advisoriesErr == nil && len(f.deadLetters) > 0 {
 		f.deadLettersNewest, f.advisoriesErr = link.NewestDeadLetters(w.js.Context(), f.deadLetters)
+		for key := range f.deadLetters {
+			if _, ok := f.deadLettersNewest[key]; !ok && f.advisoriesErr == nil {
+				// Delivered again or dropped between the two reads: left out of this look, rather than
+				// observed without a time and counted as a look (novox/hq issue 440).
+				delete(f.deadLetters, key)
+			}
+		}
 	}
 	f.handActs, f.handActsErr = w.gatherHandActs(ctx, now)
 	f.facts.taken, _, f.facts.began, f.facts.err = exportedFacts.last()