storage: outside-of-raft snapshots put consistency in jeopardy

When we send a snapshot around outside of Raft (preemptive snapshot), we don't check whether the state we're overwriting is ahead of us.
This could lead to a replica being created, acknowledging progress, and then being reset through a snapshot.

Perhaps `canApplySnapshot` is a good place to check that. The necessary information should be deducible from the respective `HardState`s.
