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.go

@@ -244,8 +244,13 @@ func MeshConsumers() []Consumer {
 			Push:           true,
 			AckWaitSeconds: 30,
 			MaxDeliver:     5,
-			Why: "the two events the mesh's own controller reacts to; after max-deliver it " +
-				"dead-letters, because an announcement it cannot act on will not become actionable",
+			// One at a time (novox/hq issue 175): acting on a merge builds for minutes, and an
+			// announcement handed over behind it must wait on the server, not time out on the
+			// client and come back to be acted on again.
+			MaxAckPending: 1,
+			Why: "the two events the mesh's own controller reacts to, one at a time; after " +
+				"max-deliver it dead-letters, because an announcement it cannot act on will not " +
+				"become actionable",
 		},
 	}
 }