@Override
  public void writeTo(AsyncWritableChannel channel, Callback<Long> callback) {
    readSet.writeTo(channel, callback);
  }
Extract the encryption key from the message at the given index from the readSet.
@param readSetIndex the index in the readSet for the message from which the encryption key has to be extracted.
@param encryptionKeyRelativeOffset the relative offset of the encryption key record in the message.
@param encryptionKeySize the size of the encryption key record in the message.
@return the extracted encryption key.
@throws IOException if an IO error is encountered while deserializing the message.
@throws MessageFormatException if a Message Format error is encountered while deserializing the message.
ambry-messageformat/src/main/java/com/github/ambry/messageformat/MessageFormatSend.java