Switch to changes table for gerrit's reviewer count file

The change_id table of our production database seems to miss the first
~6000 entries, which covers up to ~1st May 2012). Regardless of
whether this is gwtorm magic or it got corrupted at some point, we
cannot use it to get all change ids. So we switch to use the changes
table directly, eventhough this makes the query slightly more
expensive.

Bug: 52329
Change-Id: Ic15c39da0b7780baf54ba61807b13e5b1d4eb7be