Make the merges consumer before the events consumer narrows, from the old one's ack floor, so no merge is lost to the change (issue 973)

internal/broker/derived.go

@@ -36,6 +36,10 @@ type Consumer struct {
 	// them over, and a loop that acts on one message at a time is the property the store window
 	// depends on. A queue group implies this, because a group has nothing to pull from.
 	Push bool
+	// TakesOverFrom names a consumer of the same stream whose filter this one takes over (issue 973): made while
+	// that consumer still filters this one's subject, it starts after what that consumer has acknowledged, so a
+	// message the old one held unacknowledged is not lost to the change. Otherwise it starts as FromNow says.
+	TakesOverFrom string
 	// DeliverSubject, when set, is where a push consumer delivers instead of DeliverSubjectFor's.
 	DeliverSubject string
 	// AckWaitSeconds before an unacknowledged delivery is redelivered.