Add a missing DaemonLogEvent key for garbage collection

Summary: See PHI176. This is issued periodically by the garbage collector. Normally this table is relatively small-ish so this missing key isn't hugely noticeable.

Test Plan:
Ran `./bin/garbage collect --collector daemon.processes --trace` to get the query the GC runs.

Ran ##DELETE FROM `daemon_logevent` WHERE epoch < 1508443504 LIMIT 100## before and after the key, saw a much better query plan afterward:

Before:

```
mysql> explain DELETE FROM `daemon_logevent` WHERE epoch < 1508443504 LIMIT 100;
+----+-------------+-----------------+------+---------------+------+---------+------+-------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------------+------+---------------+------+---------+------+-------+-------------+
| 1 | SIMPLE | daemon_logevent | ALL | NULL | NULL | NULL | NULL | 19325 | Using where |
+----+-------------+-----------------+------+---------------+------+---------+------+-------+-------------+
1 row in set (0.00 sec)
```

After:

```
mysql> explain DELETE FROM `daemon_logevent` WHERE epoch < 1508443504 LIMIT 100;
+----+-------------+-----------------+-------+---------------+-----------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------------+-------+---------------+-----------+---------+-------+------+-------------+
| 1 | SIMPLE | daemon_logevent | range | key_epoch | key_epoch | 4 | const | 1 | Using where |
+----+-------------+-----------------+-------+---------------+-----------+---------+-------+------+-------------+
1 row in set (0.00 sec)
```

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18733

src/applications/daemon/storage/PhabricatorDaemonLogEvent.php

···
         'logID' => array(
           'columns' => array('logID', 'epoch'),
         ),
+        'key_epoch' => array(
+          'columns' => array('epoch'),
+        ),
       ),
     ) + parent::getConfiguration();
   }