Rename mBlockingDecisionsMadeUntilTime to mStateComputedTime. The invariant is that
all graph state is known up to mStateComputedTime but not beyond it (except for some
stream contents that may be buffered beyond it).
Get rid of mMessageAffectedTime and all the code around computing "affected times" and "action times".
Instead, all messages take effect at mStateComputedTime.
Get rid of the two-phase execution of messages. Everything can just happen in a single Run() method.