Change the preprocess interface for SkipperModule

[EasonLiao]

Currently the preprocess callback for SkipperModule is defined like this :

abstract ByteBuffer preprocess(ByteBuffer request);

In the preprocess callback of SkipperStateMachine, we first deserialize the ByteBuffer to command to see what types of command it is then pass ByteBuffer to preprocess of module. We should pass command directly because normally we'll deserialize the bytebuffer again in preprocess of module.