Drain the handler queues before stopping the handlers on shutdown (closes #69)

Shutdown stopped the handlers before the streamer, so updates still in the
handler queues, up to 20,000 per queue, were not written, and a lost
withdrawal left its route in the table after a restart.

Shutdown now stops the streamer first. Streamer.Stop queues the STATE
messages it holds for a full queue, closes the queues and returns once
every worker has handed its handler everything on its queue; then each
handler writes what it holds before its Stop returns. PeeringHandler.Stop
now waits for its loop's final write instead of running a second one
beside it.

Model: opus-5-5

internal/routewatch/app.go

@@ -149,7 +149,10 @@ func (rw *RouteWatch) Shutdown(ctx context.Context) {
 	rw.shutdown = true
 	rw.mu.Unlock()
 
-	// Stop batched handlers first to flush remaining batches
+	// Stop the streamer first: it returns once the handlers have been handed
+	// every message it queued. Then each handler writes what it holds.
+	rw.streamer.Stop()
+
 	if rw.dbHandler != nil {
 		rw.logger.Info("Flushing database handler")
 		rw.dbHandler.Stop()
@@ -180,9 +183,6 @@ func (rw *RouteWatch) Shutdown(ctx context.Context) {
 		rw.dbMaintainer.Stop()
 	}
 
-	// Stop services
-	rw.streamer.Stop()
-
 	// Stop HTTP server with a timeout of its own, however much of ctx is left
 	const serverStopTimeout = 5 * time.Second