CheckpointContents are the transactions included in an upcoming checkpoint.
They must have already been causally ordered. Since the causal order algorithm
is the same among validators, we expect all honest validators to come up with
the same order for each checkpoint content.
This is a message validators publish to consensus in order to sign checkpoint
The Sha256 digest of an EllipticCurveMultisetHash committing to the live object set.
Same as CheckpointContents, but contains full contents of all Transactions and
TransactionEffects associated with the checkpoint.