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)
When I was tinkering with replacements for `AppendableByteArray` I ran into this a few times. "stable" code should never reach it.