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 are being performed in input-output-hk/cardano-base#255 and input-output-hk/cardano-base#317.

To be able to work on this, the PRs mentioned above need to be merged.