Refactor snapshotting into reusable module

[ethanfrey]

Follow up on #160

Make this a storage-plus wrapper around a Map. (Bonus: ideally around indexed map as well)

Configure:

• Checkpoint every block
• Checkpoint only selected blocks (with add/remove checkpoint calls)
• Checkpoint nothing (this is just the wrapped type)

We don't really need to implement the last case, but allows us to use the same type and switch behavior based on InitMsg - as discussed here: #160 (comment)