on disk snap

## goal: 
1. save snap to disk
2. load snap from disk
3. detect corruption

## non-goal: 
1. glue with etcd.
2. async snapshotting. 
3. on disk log compact.
4. buffering and performance.
I would also like to see this named something other than `n`. That usually indicates a number of some kind. Maybe call it `name` or `fname`?