Give a consumer's max-deliveries key one watcher, counting what was given up (issue 440)

The dead-letter row and a max-deliveries advisory without a token both said
bus.<stream>.<consumer>.max-deliveries: each look added an observation and a
line of evidence through the row, and the two overwrote each other's words.
The row owns the key since issue 330, so the advisory watcher leaves it, and
the row now says when the newest held message was given up, so a letter held
for a day no longer reads as observed every 30 seconds. Times without Happened
is refused, since the raise ignored it and an update counted it.

cmd/mesh-controller/watchdogs.go

@@ -79,8 +79,11 @@ type signalFacts struct {
 	lostConsumers map[string]bool
 	// 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
-	advisoriesErr error
+	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 map[string]time.Time
+	advisoriesErr     error
 
 	selfCheck selfCheckFacts
 
@@ -350,6 +353,9 @@ func (w *watchdogs) gather(ctx context.Context) *signalFacts {
 	if f.advisoriesErr == nil && w.js != nil {
 		f.deadLetters, f.advisoriesErr = link.HeldDeadLetters(w.js.Context())
 	}
+	if f.advisoriesErr == nil && len(f.deadLetters) > 0 {
+		f.deadLettersNewest, f.advisoriesErr = link.NewestDeadLetters(w.js.Context(), f.deadLetters)
+	}
 	f.handActs, f.handActsErr = w.gatherHandActs(ctx, now)
 	f.facts.taken, _, f.facts.began, f.facts.err = exportedFacts.last()
 	return f