Use index when querying for editors in a date range

The SQL used to query for users caused a table scan [1]. By making the
implicit conversion from number to varbinary explicit, the index on
cuc_timestamp gets picked up [2]. Thereby, query times for bigger
wikis go down by ~40% (e.g.: 41 seconds -> 20 seconds for enwiki).

[1]
  EXPLAIN SELECT cuc.cuc_user, cuc.cuc_ip FROM ruwiki.cu_changes cuc WHERE cuc.cuc_namespace=0 AND cuc.cuc_user!=0 AND cuc.cuc_timestamp>20140531000000 AND cuc.cuc_timestamp<20140630000000;

yields

+------+-------------+-------+------+----------------------------------------------+------+---------+------+---------+-------------+
| id   | select_type | table | type | possible_keys                                | key  | key_len | ref  | rows    | Extra       |
+------+-------------+-------+------+----------------------------------------------+------+---------+------+---------+-------------+
|    1 | SIMPLE      | cuc   | ALL  | cuc_timestamp,cuc_user_time,cuc_user_ip_time | NULL | NULL    | NULL | 1914038 | Using where |
+------+-------------+-------+------+----------------------------------------------+------+---------+------+---------+-------------+
1 row in set (0.01 sec)

[2]
  EXPLAIN SELECT cuc.cuc_user, cuc.cuc_ip FROM ruwiki.cu_changes cuc WHERE cuc.cuc_namespace=0 AND cuc.cuc_user!=0 AND cuc.cuc_timestamp>'20140531000000' AND cuc.cuc_timestamp<'20140630000000';

+------+-------------+-------+-------+----------------------------------------------+---------------+---------+------+--------+-------------+
| id   | select_type | table | type  | possible_keys                                | key           | key_len | ref  | rows   | Extra       |
+------+-------------+-------+-------+----------------------------------------------+---------------+---------+------+--------+-------------+
|    1 | SIMPLE      | cuc   | range | cuc_timestamp,cuc_user_time,cuc_user_ip_time | cuc_timestamp | 16      | NULL | 741660 | Using where |
+------+-------------+-------+-------+----------------------------------------------+---------------+---------+------+--------+-------------+
1 row in set (0.00 sec)

Change-Id: I0667c3696666d6b5cd64a1fb9c5bcdce4c56e4a3