Add a missing key to PhabricatorFile for destroying Files

Summary: See PHI176. Depends on D18733. We issue a query when deleting files that currently doesn't hit any keys.

Test Plan:
Ran `./bin/remove destroy --force --trace F56376` to get the query.

Ran ##SELECT * FROM `file` WHERE storageEngine = 'blob' AND storageHandle = '23366' LIMIT 1## before and after the change.

Before:

```
mysql> explain SELECT * FROM `file` WHERE storageEngine = 'blob' AND storageHandle = '23366' LIMIT 1;
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows  | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
|  1 | SIMPLE      | file  | ALL  | NULL          | NULL | NULL    | NULL | 33866 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
1 row in set (0.01 sec)
```

After:

```
mysql> explain SELECT * FROM `file` WHERE storageEngine = 'blob' AND storageHandle = '23366' LIMIT 1;
+----+-------------+-------+------+---------------+------------+---------+-------------+------+------------------------------------+
| id | select_type | table | type | possible_keys | key        | key_len | ref         | rows | Extra                              |
+----+-------------+-------+------+---------------+------------+---------+-------------+------+------------------------------------+
|  1 | SIMPLE      | file  | ref  | key_engine    | key_engine | 388     | const,const |  190 | Using index condition; Using where |
+----+-------------+-------+------+---------------+------------+---------+-------------+------+------------------------------------+
1 row in set (0.00 sec)
```

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18734

src/applications/files/storage/PhabricatorFile.php

@@ -134,6 +134,9 @@ final class PhabricatorFile extends PhabricatorFileDAO
           'columns' => array('builtinKey'),
           'unique' => true,
         ),
+        'key_engine' => array(
+          'columns' => array('storageEngine', 'storageHandle(64)'),
+        ),
       ),
     ) + parent::getConfiguration();
   }