@Override
    public long progress() {
      return progress.get();
    }
Dispenses the next chunk from {@link #chunksToWrite} if one is available and adds the chunk dispensed to
{@link #chunksAwaitingCallback}.
@param allocator the {@link ByteBufAllocator} to use if required.
@return a chunk of data if one is available. {@code null} otherwise.
ambry-rest/src/main/java/com/github/ambry/rest/NettyResponseChannel.java