Refactor storage migrations

[oyvindeide]

Adding a new migration is getting a bit complicated, as we have to add a new entry and remember to add all previous migrations, as we are always going from version x -> latest, instead of iterating through the changes. Suggest a refactor to something like:

if version == 0:
    # Block storage
    migration 0 -> latest
else:
    for migration in migrations:
        migration(...)

then migrations are functions like 1to2, 2to3, or something like that.