Detect forked lineages when replicating spheres incrementally

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Currently we do some diligence to make sure that spheres most likely have a causal order when replicating them incrementally (that is, by replicating all the version deltas between two given versions).

However, we do not delve into the sphere to verify that the UCANs have fully valid proof chains and that there are no revocations associated with any of the proofs. This is necessary in order to circumvent attackers who may try to forge history for a sphere using a revoked credential.