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

src/applications/project/controller/PhabricatorProjectMoveController.php

@@ -148,6 +148,9 @@ final class PhabricatorProjectMoveController
       list($pri, $sub) = ManiphestTransactionEditor::getAdjacentSubpriority(
         $task,
         $is_after);
+
+      // If we find a priority on the first try, don't keep going.
+      break;
     }
 
     $xactions = array();