Adapt KES usage to new, non-pure, interface

[iquerejeta]

We are changing the way the KES keys are handled, to increase the guarantees that the key is not copied to disk and safely deleted by mlocking the secret data. These changes have modified the KES interface, which use to expose pure functions, to a non-pure interface.

The changes have been performed in cardano-base

https://github.com/input-output-hk/cardano-base/pull/255

and we should plan how to synchronise the required changes in the consensus codebase.