Snapshot file compaction and truncation

Currently all snapshot files must be kept forever.
What we should have is a feature to occasionally migrate "ancient" snapshot pages to new snapshots so that we can delete old snapshot files.

We need some kind of statistics to see when we should trigger it, and have to think about the API to expose the feature. This a big TODO.

In the meantime, it might be okay to have a tentative "compact-all" feature to migrate all snapshot pages to a single new snapshot, and delete all snapshot files.