The controller takes one announcement at a time

A merge's handler builds for minutes and keeps its own delivery alive; the announcements handed over
behind it timed out on the client and came back, and a merge that came back rebuilt what it had just
built, five times over (novox/hq issue 175). MaxAckPending 1 on the events consumer: the server holds
the rest.

internal/broker/streams_test.go

@@ -260,3 +260,14 @@ func TestNoTwoConsumersDeliverOntoTheSameSubject(t *testing.T) {
 		seen[subject] = c.Name + " on " + c.Stream
 	}
 }
+
+// The controller's events consumer is handed one announcement at a time (novox/hq issue 175): a
+// merge's handler builds for minutes, and what is queued behind it must wait on the server rather
+// than time out on the client and be acted on twice.
+func TestTheControllerTakesOneAnnouncementAtATime(t *testing.T) {
+	for _, c := range MeshConsumers() {
+		if c.Stream == "EVENTS" && c.Name == ControllerName && c.MaxAckPending != 1 {
+			t.Fatalf("the events consumer may have %d outstanding; one announcement at a time", c.MaxAckPending)
+		}
+	}
+}