Add functions to manage stored manifests

[ggilder]

Per #36, we need user-accessible functions to manage stored manifests.

Should include:

• List (show paths we have manifests for, show how many and what timestamps)
• Remove (remove a particular manifest, or all for a given path)
• Prune (remove manifests for paths that no longer exist)
• Rotate? not the right name (delete old manifests logrotate style — needs config for how many/how long to keep)

"rotate" could be automatically triggered every time the program is run