Replace AppendableByteArrayInputStream in DirectDruidClient

* Replace with SequenceInputStream fueled by an enumeration of ChannelBufferInputStream which directly wrap the response context ChannelBuffer
* Added zero-length byte array when DirectDruidClient is done. This prevents an odd race condition on `done`.

(old pull request didn't like the force)
It is not passed into this function. This function ensures that once DONE is set, there is at least 1 more empty buffer in the stream.