First stub of the feedback API.

First stub of the feedback API.

FRSH Feedback is implemented in this commit. It only is in place for
AQuoSA based CPU resource manager, since CGroups still lacks the basics
for having it up and running (it can be done, but it will be the focus
of future work... Not enough time to deal with it now!).

Something is still missing, like retreival and renegotiation of the fake
spare_contract used for the feedback. Some more discussion is needed in
order of deciding how to realize it.