Share some stuffs with the initial reader when new stream reader created

When switching pread to stream read, new HFileReaderImpl will be create, but the two different readers do not share informations with each other. maybe we can divide HFileReaderImpl into two different class, such as HFilePreadReader and HFileStreamReader. When constructing HFileStreamReader, it will copy some stuffs (fileInfo, index, etc) from an already existing Reader, and no need to do prefetch operations.