Harmonize callbacks interfaces (250)

[MathisGD]

Attempt to fix #245, with the following rationale:

• we should not pass data that can be known by the user at low cost
• we put amount everywhere because it is mandatory in repay and liquidate, for harmonization

[Rubilmax]

I believe we're removing callback parameters that cost 100s of gas passing while it can greatly improve some (but not all, perhaps not even the majority of) integrators' lives... I really don't get where we're going

[MathisGD]

Closing as Rubilmax, still one of the biggest Blue's integrator, is against

[Rubilmax]

This code works, but I didn't change my mind:

we're removing callback parameters that cost 100s of gas passing while it can greatly improve some (but not all, perhaps not even the majority of) integrators' lives

EDIT: it's not a big deal