Reduce unnecessary snapshots by storing local snapshot metadata

[marcua]

• Right now we dump a database on each periodic snapshot run
• We then hash the database, and if the hash exists in S3-compatible storage, we don't upload it
• We can avoid having to snapshot/hash by storing ephemeral metadata on the last time we ran snapshotting: if no file was modified since our last snapshot run, don't dump the database/hash it at al. More technically, get fs::metadata of each file in the dir, call modified() on result, sort the times so it's stable, take max or hash them together. If the max/hash of the modified times hasn't changed, skip this backup.