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/peeringhandler_internal_test.go

@@ -188,6 +188,22 @@ func TestPeeringHandlerProcessesEachRunAndEmpties(t *testing.T) {
 	}
 }
 
+// TestPeeringHandlerStopWritesTheRemainingPaths hands a running handler one
+// path and stops it. The peering must be in the database when Stop returns.
+func TestPeeringHandlerStopWritesTheRemainingPaths(t *testing.T) {
+	t.Parallel()
+
+	store := &recordingStore{mockStore: newMockStore()}
+	h := NewPeeringHandler(store, logger.New())
+
+	handle(h, time.Now().UTC(), testASNA, testASNB)
+	h.Stop()
+
+	if got := store.callCount(); got != 1 {
+		t.Errorf("want 1 RecordPeering call when Stop returns, got %d", got)
+	}
+}
+
 // TestPeeringHandlerCapDropsAndCounts verifies that a full map drops new paths
 // and counts them, while a path already tracked is refreshed rather than
 // dropped.