issue-324: DescribeData: 1. BlobOffsets should be in bytes, not blocks 2. returning FileSize 3. added a ut that checks that we don't return data outside of the aligned superrange that contains the requested range

issue-324: DescribeData: 1. BlobOffsets should be in bytes, not blocks 2. returning FileSize 3. added a ut that checks that we don't return data outside of the aligned superrange that contains the requested range #526