Prevent "lazy" validator strategy

[karimodm]

If a validator wants to be lazy, it could simply observe the bahavior of an honest other and select its blocks as parents, copying the commitments into its own blocks.

To possibly prevent this we could alter the commitment so that the hash would be: H(IdentityID + MerkleRoot) ; this hash would be signed by the privatekey of the IdentityID seeding the hash. In this way a lazy actor cannot simply copy the commitment, as he would need to seed its own MerkleRoot (now obscured away) with its own Identity and sign it.