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, we'd already fatal when trying to move tasks to the archive table. Just clean this up for consistency, and remove the ancient codepath which imagined tasks with no data.

Test Plan:

• Ran bin/storage upgrade, inspected tables.
• Ran bin/phd debug taskmaster, worked through a bunch of tasks with no problems.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13253, T6615

Differential Revision: https://secure.phabricator.com/D20175