Add a unique key to OwnersPath on "<packageID, repositoryPHID, pathIndex>"

Summary:
Depends on D19181. Ref T11015. This nukes duplicates from the table if they exist, then adds a unique key.

(Duplicates should not exist and can not be added with any recent version of the web UI.)

Test Plan:
- Tried to add duplicates with web UI, didn't have any luck.
- Explicitly added duplicates with manual `INSERT`s.
- Viewed packages in web UI and saw duplicates.
- Ran migrations, got a clean purge and a nice unique key.
- There's still no way to actually hit a duplicate key error in the UI (unless you can collide hashes, I suppose), this is purely a correctness/robustness change.

Maniphest Tasks: T11015

Differential Revision: https://secure.phabricator.com/D19182

resources/sql/autopatches/20180306.opath.03.purge.php

···
+<?php
+
+$table = new PhabricatorOwnersPath();
+$conn = $table->establishConnection('w');
+
+$seen = array();
+foreach (new LiskMigrationIterator($table) as $path) {
+  $package_id = $path->getPackageID();
+  $repository_phid = $path->getRepositoryPHID();
+  $path_index = $path->getPathIndex();
+
+  if (!isset($seen[$package_id][$repository_phid][$path_index])) {
+    $seen[$package_id][$repository_phid][$path_index] = true;
+    continue;
+  }
+
+  queryfx(
+    $conn,
+    'DELETE FROM %T WHERE id = %d',
+    $table->getTableName(),
+    $path->getID());
+}

resources/sql/autopatches/20180306.opath.04.unique.sql

···
+ALTER TABLE {$NAMESPACE}_owners.owners_path
+  ADD UNIQUE KEY `key_path` (packageID, repositoryPHID, pathIndex);

src/applications/owners/storage/PhabricatorOwnersPath.php

···
         'excluded' => 'bool',
       ),
       self::CONFIG_KEY_SCHEMA => array(
-        'packageID' => array(
-          'columns' => array('packageID'),
+        'key_path' => array(
+          'columns' => array('packageID', 'repositoryPHID', 'pathIndex'),
+          'unique' => true,
         ),
       ),
     ) + parent::getConfiguration();