Split update_holder_per_commitment

Split update_holder_per_commitment

Split `update_holder_per_commitment` into two parts:

1. `update_holder_per_commitment_point`, which we call to retrieve a new
   commitment point.
2. `update_holder_commitment_secret`, which we call when we're ready to
   release the commitment secret.

This delays releasing the secret until we actually need it for the
revoke-and-ack.

By doing this, we restore the signer check to its original condition, as well.