ContribsPager: Fix slow queries

ContribsPager: Fix slow queries

When ContribsPager is using an auxiliary table like ip_changes or
revision_actor_temp for the main action of the query, we already had
code in place to let it use the auxiliary table's denormalized timestamp
field for the ordering. What we didn't have was code to let it also use
the auxiliary table's denormalized timestamp field for *continuation*.

With the schema defined in tables.sql, the simplest thing to do would be
to be to add a redundant JOIN condition between rev_timestamp and the
denormalized timestamp field which would be enough to allow
MySQL/MariaDB to propagate the continuation conditional on rev_timestamp
to the denormalized timestamp field.

Unfortunately many Wikimedia wikis have rev_timestamp defined
differently from table.sql (P8433), and that difference is enough to
break that propagation. So we need to take a more difficult route,
restructuring the code tell IndexPager to explicitly use the
denormalized fields for ordering and continuation.

On the plus side, since we're doing that anyway we can get rid of the
code mentioned in the first paragraph.

Bug: T221380
Change-Id: Iad6c0c2f1ac5e1c610de15fe6e85a637c287bcd8