WIP: Baker: use deterministic nonces

Currently, when the PoS requires the baker to commit the hash of an entropy nonce in a block, the implementation generates some sequence of bytes from a system random source, hashes it to produce the commitment, and saves it in a file for revealing it when the time comes. This is bad because it could lead to bonds loss in case the machine crashes, the file is lost, and the baker is unable to reveal its nonce.

In this MR, the baker now uses the signer's deterministic_nonce and deterministic_nonce_hash as a way of computing seed nonces, whenever these are implemented. That is, the baker handles both the old and new commitment schemes.