Fix checkpoint bootstrapping to account for trying to recover between checkpoints

[Hellblazer]

Analysis of checkpoint bootstrapping flappers has shown that it's easily possible for a bootstrapping node to start bootstrapping on the most recent checkpoint A (from group POV) and in the process of recovering another checkpoint B will occur in the system. This wipes out the blocks between checkpoints A and B, and the current Boostrapper fails to recover - it simply keeps on trying to synchronize these blocks which no longer exists.