Change Title::getPreviousRevisionID (and next) to ignore PRIMARY

Both Title::getPreviousRevisionID and Title::getNextRevisionID
have bad performance under certain versions of MySQL/MariaDB
when the page has many revisions (in the order of dozens of
thousands). This is good enough in most cases.

However, on a contributions slave, where it has an explicity
defined extended secondary key with the primary key, the
performance is really, really bad, performing a full table scan.

By ignoring the PRIMARY KEY index, contributions slaves go from the
worst case to the best case, while not affecting the plan of regular
slaves. It is believed that more recent versions of MySQL chose the
right indexes automatically, because they can use the extended primary
key automatically, without adding it explicitly. If that doesn't
happen, we can consider adding it explicitly to the regular slaves,
too.

In any case, not using the PRIMARY gives always a much better or at
least the same performance (this never causes a regression, unlike
FORCE'ing a specific index).

Bug: T159319
Change-Id: Ibb6e5240b87bd8e2d680fc4d58fcb3db1a4721cc