Store snapshot page progress and resume from a specific snapshot page

Currently, pages of snapshots are re-downloaded each time, even though the pages included in those snapshots are already in the cache. We should be able to resume progress from a specific page of snapshots, rather than re-downloading as we do now.

Start by tracing how snapshot pages are downloaded and how cached pages are checked. Define how progress for a specific snapshot page is stored and resumed; done means rerunning the utility skips already cached snapshot pages instead of downloading them again.

Written by the indexing model from the issue text.