Fold the datastore version into the Thar image version

[sam-aws]

Since a datastore version update always corresponds to a Thar image update, and a single Thar version is unlikely (or never going) to have multiple associated datastore versions, things could be simplified by getting rid of the datastore version entirely.

This would have a number of knock-on effects:

• updater/* tools need to change to use the Thar version for migrations
• update_metadata needs to be updated to remove mappings
• migrations / migration install handling needs to update its versioning scheme
• API tools need to update to use the Thar version where applicable
• a transition period is required where tools and manifest.json support both schemes, before a point in time where any older image versions are deprecated and upgrading from them is not possible.

[iliana]

Fixed in #760.