Add index on delayed_jobs (locked_at, locked_by)

This index avoids an increasingly growing performance penalty when delayed_jobs table tends to grow bigger.

Before index:
```
mysql> explain SELECT  `delayed_jobs`.* FROM `delayed_jobs`  WHERE `delayed_jobs`.`locked_at` = '2013-08-13 13:03:06' AND `delayed_jobs`.`locked_by` = 'delayed_job host:private.internal pid:12345' LIMIT 1;
+----+-------------+--------------+------+---------------+------+---------+------+--------+-------------+
| id | select_type | table        | type | possible_keys | key  | key_len | ref  | rows   | Extra       |
+----+-------------+--------------+------+---------------+------+---------+------+--------+-------------+
|  1 | SIMPLE      | delayed_jobs | ALL  | NULL          | NULL | NULL    | NULL | 260546 | Using where |
+----+-------------+--------------+------+---------------+------+---------+------+--------+-------------+
```

After:
```
mysql> explain SELECT  `delayed_jobs`.* FROM `delayed_jobs`  WHERE `delayed_jobs`.`locked_at` = '2013-08-13 13:03:06' AND `delayed_jobs`.`locked_by` = 'delayed_job host:private.internal pid:12345' LIMIT 1;
+----+-------------+--------------+------+-------------------+-------------------+---------+-------------+------+-------------+
| id | select_type | table        | type | possible_keys     | key               | key_len | ref         | rows | Extra       |
+----+-------------+--------------+------+-------------------+-------------------+---------+-------------+------+-------------+
|  1 | SIMPLE      | delayed_jobs | ref  | delayed_jobs_lock | delayed_jobs_lock | 312     | const,const |    1 | Using where |
+----+-------------+--------------+------+-------------------+-------------------+---------+-------------+------+-------------+
```

Visual performance impact after this migration was applied to a production app during rush hour:
![preview](http://new.tinygrab.com/bd43eb7ea95c6a4eb72e94648064502f3ed39bb949.png)

Kudos to @tomas-didziokas for suggesting this fix.