Avoid redundant VerifyDealsForActivation call in ProveReplicaUpdates

[anorth]

ProveReplicaUpdates both activates deals and then (afterwards!) invokes VerifyDealsForActivation in order to retrieve the (computed) sector data commitments. This involves two call overheads, and loading the deals from state twice.

Combine these into one call by returning all necessary information from ActivateDeals.

[anorth]

We probably want a flag for ActivateDeals specifying whether to compute CommD, because it's needed only for replica update, not prove-commit (where CommD is already known).