BaseDiskFileWriter will track md5 and expose upload_size, etag via the
chunks_finished method.
The BaseDiskFileReader already tracks the md5/etag via _iter_etag, for
parity we add a _chunks_etag to the BaseDiskFileReader.
Instead of returning the upload_size and hexdigest every call to write,
we return the tuple from chunks_finished.
Change-Id: I26c58719cff5fde941d0248c250a0204e0379ae5