This sub-task addresses section 4.1 of the document attached to HDFS-5194,
the exposure of interfaces needed by a FsDatasetSpi implementation.
Specifically it makes ChunkChecksum public and BlockMetadataHeader's
readHeader() and writeHeader() methods public.
The changes to BlockReaderUtil (and related classes) discussed by section
4.1 are only needed if supporting short-circuit, and should be addressed
as part of an effort to provide such support rather than this JIRA.
To help ensure these changes are complete and are not regressed in the
future, tests that gauge the accessibility (though not behavior)
of interfaces needed by a FsDatasetSpi subclass are also included.
These take the form of a dummy FsDatasetSpi subclass – a successful
compilation is effectively a pass. Trivial unit tests are included so
that there is something tangible to track.