R1:4b10bc2b643d

Correct schema irregularities (including weird keys) with worker task tables

Summary:
Ref T13253. Fixes T6615. See that task for discussion.

- Remove three keys which serve no real purpose: `dataID` doesn't do anything for us, and the two `leaseOwner` keys are unused.
- Rename `leaseOwner_2` to `key_owner`.
- Fix an issue where `dataID` was nullable in the active table and non-nullable in the archive table.

In practice, //all// workers have data, so all workers have a `dataID`: if they didn't…