ContribsPager: Disallow looking too far in the past for 'newbies' queries

ContribsPager: Disallow looking too far in the past for 'newbies' queries

If the user requested a timestamp offset far in the past such that
there are no edits by users with user_ids in the range, we would end
up scanning all revisions from that offset until start of time.

This might end up generating funny queries with redundant conditions
on rev_timestamp, but that should not be a problem, and trying to
tweak paging logic would probably be more difficult than this.

Bug: T140537
Change-Id: I2ac9abee09529620588923bbafbcac07ebe466b2