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`.
* Improves query speeds by 1~2% in my simple tests

(old pull request didn't like the force)
I was concerned AtomicBoolean might do synchronized(this) calls in it somewhere, but the exposed code does no such thing, so I'm implementing the change you suggest.