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.

internal/conditions/condition.go

@@ -154,7 +154,9 @@ type Condition struct {
 	Gaps  []Gap     `json:"gaps,omitempty"`
 	// Observations is how many times it was observed since raised: a look that sees it, for a source
 	// that looks at the present; for one that reads a record of what happened (Observation.Happened),
-	// how many times it happened, and LastObserved when it last did (novox/hq issue 402).
+	// how many times it happened, and LastObserved when it last did (novox/hq issue 402). That count is a
+	// running total since raised, never lowered: a consumer's dead letters count each message given up on
+	// while the condition is open, not the number held now (novox/hq issue 440).
 	Observations int `json:"observations"`
 	// Count is how many times it has been raised, a reopening within ReopenWithin counted.
 	Count    int       `json:"count"`