When rewinding, remove insufficiently-validated blocks
If a block is insufficiently-validated against a particular branch ID, then we
cannot guarantee that even the block header will be valid under the actual
consensus rules the node will want to apply. Instead require that the blocks are
completely re-validated, by removing them from the block index (which is
equivalent to reducing their validity to BLOCK_VALID_UNKNOWN).
Closes#3100.
3a47b9bcfd