Migrate atom ids to UUIDs

[joshsh]

Newly-minted atom ids in SmSn are currently 16 base-62 characters, or around 96 bits. Let's increase that to 128 bits, and make atom ids into base-62 encoded UUIDs. That should rule out any future concerns about id collisions, and allow commonplace tools to be used for working with atom ids.

[joshsh]

Important: before performing the migration, for each graph, create a file which preserves the mapping between the original ids and the new UUIDs, as a historical record. Older activity logs will become meaningless without this mapping. Alternatively or in addition, simply rewrite the activity logs.