MantaClient#listObjects leaks a MantaDirectoryListingIterator when the iterator is empty

[tjcelaya]

Following some investigation from #398 led to the discovery that some directory listing operations also leak connections. Currently disentangling the BufferedReader close from the CloseableHttpResponse close to see where exactly this is occurring.

This seems to be more relevant to the parallelism of the findForkJoinPool when setting max_connections = 1 and is not actually a connection leak. Renamed.

The issue was in Mantaclient#listObjects intentionally returning an empty stream but forgetting to close the iterator used to determine the stream should be empty.