[cli] agentblame.db migrations

[spencatro-pub]

There is not currently a way to "migrate" existing agent blame databases (e.g. from v2 to v3). The next time the database changes, it would be great if agent blame used some kind of migration system so that data tracked by older versions isn't lost. (This means I don't have a good way to submit provenance data for #36 , for example.)

(Probably not worth doing for v2->v3, except maybe as an exercise to verify any new migration system)

[muraalee]

Totally agree. I considered it, but the changes were significant and since v2 never had broad adoption, I decided to go with this approach instead.