Avoid creating lots and lots of cat_id gaps

Avoid creating lots and lots of cat_id gaps

Currently, INSERT...ON DUPLICATE KEY UPDATE is used to update the page
counts in the category table. However, MySQL 5.1.22 and newer, by default,
increment the counter for cat_id before checking for duplicate key errors.
This creates many gaps in the cat_id sequence.

To avoid this, check for existing category rows, and instead UPDATE any
that were found. It is hoped that the extra queries will not significantly
harm performance.

Change-Id: Ic2ab9ff14f04a0c7ea90a5b6756cade0c78e2885