Improve use of index for fetching most recent feedback activity from logging table

Previous code (on current enwiki.logging)
+------+-------------+---------+------+---------------+-----------+---------+-------+---------+----------------------------------------------+
| id   | select_type | table   | type | possible_keys | key       | key_len | ref   | rows    | Extra                                        |
+------+-------------+---------+------+---------------+-----------+---------+-------+---------+----------------------------------------------+
|    1 | SIMPLE      | logging | ref  | page_time     | page_time | 4       | const | 1330612 | Using where; Using temporary; Using filesort |
+------+-------------+---------+------+---------------+-----------+---------+-------+---------+----------------------------------------------+

This patch (on current enwiki.logging)
+------+-------------+---------+-------+---------------+-----------+---------+------+------+---------------------------------------------------------------------+
| id   | select_type | table   | type  | possible_keys | key       | key_len | ref  | rows | Extra                                                               |
+------+-------------+---------+-------+---------------+-----------+---------+------+------+---------------------------------------------------------------------+
|    1 | SIMPLE      | logging | range | page_time     | page_time | 261     | NULL |   76 | Using index condition; Using where; Using temporary; Using filesort |
+------+-------------+---------+-------+---------------+-----------+---------+------+------+---------------------------------------------------------------------+

Change-Id: Ib6755276a561a30bee1feadd9df5a8442b1ef6e8
Merge "Improve use of index for fetching most recent feedback activity from logging table"