src/backup/data_blob.rs: new load_from_reader(), which verifies the CRC

And make verify_crc private for now. We always call load_from_reader() to
verify the CRC.

Also add load_chunk() to datastore.rs (from chunk_store::read_chunk())

src/client/remote_chunk_reader.rs

@@ -42,8 +42,9 @@ impl RemoteChunkReader {
             .download_chunk(&digest, &mut chunk_data)
             .await?;
 
-        let chunk = DataBlob::from_raw(chunk_data)?;
-        chunk.verify_crc()?;
+        let chunk = DataBlob::load_from_reader(&mut &chunk_data[..])?;
+        
+        // fixme: verify digest?
 
         Ok(chunk)
     }