Shutdown FetcherThreads when closing Hdf5ImageLoader

src/main/java/bdv/img/hdf5/Hdf5ImageLoader.java

@@ -235,6 +235,7 @@ public class Hdf5ImageLoader implements ViewerImgLoader, MultiResolutionImgLoade
 					return;
 				isOpen = false;
 
+				fetchers.shutdown();
 				cache.clearCache();
 				hdf5Access.closeAllDataSets();