Fixes grpc server starving. (#238)
Fixes grpc server starving. (#238)

#224 introduced an RwLock. The grpc server potentially waited forever for write access, since the read lock wasn't released. This PR fixes this bug.
cartographer-project/point_cloud_viewer,cartographer-project/point_cloud_viewer,googlecartographer/point_cloud_viewer,googlecartographer/point_cloud_viewer,cartographer-project/point_cloud_viewer,googlecartographer/point_cloud_viewer,cartographer-project/point_cloud_viewer,googlecartographer/point_cloud_viewer,cartographer-project/point_cloud_viewer,googlecartographer/point_cloud_viewer