public ByteString toByteString() {
      // We need this copy to create the ByteString as the byte[] 'buf' is not immutable. We reuse
      // them.
      return ByteString.copyFrom(this.buf, 0, this.count);
    }
Create and setup a {@link WALCellCodec} from the CompressionContext. Cell Codec classname is
read from {@link Configuration}. Fully prepares the codec for use.
@param conf        {@link Configuration} to read for the user-specified codec. If none is
                   specified, uses a {@link WALCellCodec}.
@param compression compression the codec should use
@return a {@link WALCellCodec} ready for use.
@throws UnsupportedOperationException if the codec cannot be instantiated
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/wal/WALCellCodec.java