Simplify CommonService interface

Simplify CommonService interface

Introduce utility RequestCallback classes, which encapsulate common
FutureCallbackFunctionality.

Instead of transforming functions, introduce sendRequest() abstract
method. This method is invoked once the request has been allocated and
it is responsible for building the request payload and calling
commitRequest() with the payload and appropriate callback.

Also introduce AbstractVoidService, which maps responses to Void and
ensures the slot is always committed. It also enqueues a
VoidFutureCallback to complete the request.

AbstractSimpleService handles remapping of result to the appropriate
class, if it is present.

AbstractMultipartService handles gathering of multipart requests.

Change-Id: I047ff873f09a5dd7e95d8dc101429f7c3ee248a1
Signed-off-by: Robert Varga <rovarga@cisco.com>