Fix /sync when we have no events (#341)

* Fix /sync when we have no events

We used a since token of 0 to mean that no token was given. However, naffka
streams start at 0. This causes clients to get stuck spinning forever until an
event is sent.

This changes it so that we pass around pointers instead, with nil meaning a
since token wasn't given.

* Comment

* Fix unit tests

* Comments

* Fix typo

src/github.com/matrix-org/dendrite/syncapi/sync/notifier_test.go

@@ -264,7 +264,7 @@ func waitForEvents(n *Notifier, req syncRequest) (types.StreamPosition, error) {
 		return types.StreamPosition(0), fmt.Errorf(
 			"waitForEvents timed out waiting for %s (pos=%d)", req.userID, req.since,
 		)
-	case <-listener.GetNotifyChannel(req.since):
+	case <-listener.GetNotifyChannel(*req.since):
 		p := listener.GetStreamPosition()
 		return p, nil
 	}
@@ -282,7 +282,7 @@ func newTestSyncRequest(userID string, since types.StreamPosition) syncRequest {
 	return syncRequest{
 		userID:        userID,
 		timeout:       1 * time.Minute,
-		since:         since,
+		since:         &since,
 		wantFullState: false,
 		limit:         defaultTimelineLimit,
 		log:           util.GetLogger(context.TODO()),