@Override
    public HttpContent readChunk(ChannelHandlerContext ctx) throws Exception {
      return readChunk(ctx.alloc());
    }
Dispenses the next chunk from {@link #chunksToWrite} if one is available and adds the chunk dispensed to
{@link #chunksAwaitingCallback}.
@param ctx the {@link ChannelHandlerContext} for the channel being written to.
@return a chunk of data if one is available. {@code null} otherwise.
ambry-rest/src/main/java/com/github/ambry/rest/NettyResponseChannel.java