Add index on thread_id, created to {pm_index}

SELECT uid FROM pm_index WHERE thread_id = ? AND created is NULL isn't indexed.

mysql> EXPLAIN SELECT uid FROM pm_index WHERE thread_id = 2 AND created IS NULL;
---------------+----------+------+-----------------------------------------+--------------+---------+-------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
---------------+----------+------+-----------------------------------------+--------------+---------+-------+------+-------------+ | 1 | SIMPLE | pm_index | ref | participants,thread_id,pm_index_created | participants | 4 | const | 1 | Using where |
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The query came up in NewRelic as taking over 400ms (with a different thread_id - I believe there were a large numbers of users on the thread).