DfsLogger.readHeaderAndReturnStream should return a Closeable

With the change in https://github.com/apache/accumulo/pull/369, it reveals that there's a possible unclosed resource hidden in the object returned from DfsLogger.readHeaderAndReturnStream. This should return something which is a Closeable, so it can be used in a try-with-resources block and cleaned up properly just in case the extra stream allocates additional resources which need to be closed.