Simplify DOMOperation{Service,Implementation}

Simplify DOMOperation{Service,Implementation}

In order to seamlessly bridge Binding->DOM->Binding invocation
without incurring thread safety hits we need to have a way
for the operation implementations to give us a handle.

We therefore should not be bridging the result through a callback,
but rather via a FluentFuture -- eliminate DOMOperationCallback
and make the former bridge methods the only API.

Change-Id: I3d1064b95ca65b44909b27bf028d050594ecbee7
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>