Correct schema irregularities (including weird keys) with worker task tables
- 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.
- Ran bin/storage upgrade, inspected tables.
- Ran bin/phd debug taskmaster, worked through a bunch of tasks with no problems.
Reviewed By: amckinley
Differential Revision: https://secure.phabricator.com/D20175