Make HFileReaderImpl.getFirstKeyInBlock() to return a Cell

      if (reader.getComparator()
          .compareKeyIgnoresMvcc(
              new KeyValue.KeyOnlyKeyValue(firstKey.array(), firstKey.arrayOffset(),
                  firstKey.limit()), key) >= 0) {
        long previousBlockOffset = seekToBlock.getPrevBlockOffset();
        // The key we are interested in
        if (previousBlockOffset == -1) {
          // we have a 'problem', the key we want is the first of the file.
          return false;
        }
........

      Cell firstKeyInCurrentBlock = new KeyValue.KeyOnlyKeyValue(Bytes.getBytes(firstKey));
      loadBlockAndSeekToKey(seekToBlock, firstKeyInCurrentBlock, true, key, true);