Avoid marking blocks complete prematurely while validating

[piotr-dziubecki]

We need to ensure for a validating node that it doesn't mark a block complete immediately after executing it (as it doesn't have strict finality yet) and likewise it shouldn't notify the block accumulator at that point. Current idea is to handle that in the ReactorAnnouncement::CompletedBlock section of dispatch_event, and also to move the section under BlockAccumulatorAnnouncement::AcceptedNewBlock there too.