epoch-based event log truncation

epoch-chop.md

• yes to epoch system
• each epoch has a version file, an initial snapshot, and an event log database file
• we will need to add code to vere to find the latest epoch folder before loading snapshot and event log
• there are two separable operations involved in truncation:

1. create new epoch, by copying snapshot into a new epoch folder, creating a new version file in that folder, and creating a new event log database file in that folder whose events db is empty and whose metadata db contains entries copied from the previous epoch's metadata db
2. a "keep only N recent epochs" command, which removes all epoch folders older than that