Migration: handle data store format changes

[tjkirch]

i.e. "major" version changes, for example if we change from a filesystem data store to SQLite or LMDB.

• the migrator has to run migrations for major-version transitions (because we need to finish any minor version migrations before the major, and there may be more minor after the major)
• the migrator has to handle major version links
• research other changes that need to be made

[iliana]

@tjkirch tells me we don't have a "data store format" anymore so this no longer applies. 🎉