Drop uniqueness constraint on PushEvent request ID

Summary: See <https://discourse.phabricator-community.org/t/pushing-to-mercurial-repository-fails/1275/1>. Mercurial may invoke hooks multiple times per push.

Test Plan: Pushed to Mercurial, saw key constraint failure.

Differential Revision: https://secure.phabricator.com/D19257

resources/sql/autopatches/20180326.lock.03.nonunique.sql

···
+ALTER TABLE {$NAMESPACE}_repository.repository_pushevent
+  DROP KEY `key_request`;

src/applications/repository/storage/PhabricatorRepositoryPushEvent.php

···
         'key_repository' => array(
           'columns' => array('repositoryPHID'),
         ),
-        'key_request' => array(
+        'key_identifier' => array(
           'columns' => array('requestIdentifier'),
-          'unique' => true,
         ),
       ),
     ) + parent::getConfiguration();