Improve large bytes request handling by detecting content composite buff...

...er

There is a special type of request that tries to not allocate another buffer when sending bytes request (used by the public cluster state action). With the new pages bytes reference support, the content can already be a composite channel buffer, take that into account when building the actual composite buffer that will be sent over the network
maye we should have a static method or a Ctor on the `CompositeChannelBuffer` that can do this kind of optimization?