Serve the read verbs on the serving controller's own connection, and name every connection

Each verb ran as a process that dialled the bus, so hundreds of short
connections an hour, all named mesh-controller, hid any client reconnecting
in a loop (hq issue 327). D15 now says a user whose connections keep dropping.

cmd/mesh-controller/build.go

@@ -879,6 +879,10 @@ func heldBy(ctx context.Context) map[string]string {
 // the mesh runs on today this needs the controller's own connection, so it is handed one; on the bus
 // being built it dials, because a build request is a one-shot and holds nothing else.
 func askOverOn(seat string) (link.Builders, error) {
+	// The serving controller asks on its own connection (novox/hq issue 327).
+	if serving := servingBus.Load(); serving != nil {
+		return link.BuildsOn(serving, seat), nil
+	}
 	address, err := broker.BusAddress()
 	if err != nil {
 		return nil, err
@@ -937,11 +941,7 @@ func buildSeatAmong(entries []inventory.Entry) string {
 // with a consumer of its own that is gone when this returns, so nothing accumulates in the server
 // for the reading, and filtered by subject, so one build's lines are all that travel.
 func buildLog(ctx context.Context, id string) error {
-	address, err := broker.BusAddress()
-	if err != nil {
-		return err
-	}
-	js, err := broker.Dial(address)
+	js, err := aBus()
 	if err != nil {
 		return fmt.Errorf("cannot reach the bus to read a build's log: %w", err)
 	}