Disperse task subpriorities in blocks

Summary:
Ref T7664. The current algorithm for moving task subpriorities can end up stuck in a real sticky swamp in some unusual situations.

Instead, use an algorithm which works like this:

• When we notice two tasks are too close together, look at the area around those tasks (just a few paces).
• If things look pretty empty, we can just spread the tasks out a little bit.
• But, if things are still real crowded, take another look further.
• Keep doing that until we're looking at a real nice big spot which doesn't have too many tasks in it in total, even if they're all in one place right now.
• Then, move 'em out!

Also:

• Just swallow our pride and do the gross INSERT INTO ... "", "", "", "", "", "", ... ON DUPLICATE KEY UPDATE to bulk update.
• Fix an issue where a single move could cause two different subpriority recalculations.

Test Plan:

• Changed ManiphesTaskTestCase->testTaskAdjacentBlocks() to insert 1,000 tasks with identical subpriorities, saw them spread out in 11 queries instead of >1,000.
• Dragged tons of tasks around on workboards.
• Ran unit tests.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7664

Differential Revision: https://secure.phabricator.com/D17959