Make Raw{Request,Response} ByteBufHolders

[willsalz]

We get some nice benefits e.g. automatic object releasing by ChannelHandlers ( which can bite you if you don't know it's happening! ), and a nicer API for releasing payloads e.g. RawRequest.release() instead of RawRequest.getArg1().release(); RawRequest.getArg2().release(); RawRequest.getArg2().release().

I had this implemented by wrapping the 3 contained ByteBufs in a CompositeByteBuf, but I wasn't sure about the retain/release semantics of CompositeBuffers vs. raw ByteBufs so I took out the logic. This isn't a perf. win, but may simplify retain/release APIs down the road.