feat: fix #1805 by implementing Nakamoto consensus on the history of …

…anchor blocks. The canonical Stacks fork must pass through the longest history of anchor blocks (by number of anchor blocks and empty reward cycles). Use anchor block affirmation maps to identify and track the heaviest anchor block history, and if the heaviest affirmation map changes, invalidate sortitions and reprocess them, but this time, use the new heaviest affirmation map to deduce which anchor blocks *must exist*. This not only makes it possible to reorg the Stacks blockchain if the network loses an anchor block, but also makes the act of re-affirming an anchor block N reward cycles ago *at least as hard as* mining N+1 new reward cycles.