We can now atomically switch chains when checkpointing. Previously,
we'd call `SetHead` followed by `SetCheckpoint`. Unfortunately, that's
not atomic and the "head" could have reverted before we called
`SetCheckpoint` (causing the latter to fail).
Now, we just call `SetCheckpoint` and let `SetCheckpoint` adjust our
head. This changes the behavior of `ChainStore.SetCheckpoint`, but
`Syncer.SyncCheckpoint` is the only caller anyways.