Fix an issue where internal paging of notifications could fail if some notifications are not visible

Summary:
Ref T13266. See https://discourse.phabricator-community.org/t/notification-page-throws-unrecoverable-fatal-error/2651/.

The "notifications" query currently uses offset paging for no apparent reason (just a legacy issue?), so some of the paging code is only reachable internally.

• Stop it from using offset paging, since modern cursor paging is fine here (and Feed has used cursor paging for a long time).
• Fix the non-offset paging to work like Feed.

Also:

• Remove a couple of stub methods with no callsites after cursor refactoring.

Test Plan:

• Set things up so I had more than 100 notifications and some in the first 100 were policy filtered, to reproduce the issue (I just made FeedStory return NO_ONE as a visibility policy).
• Applied the patch, notifications now page cleanly.
• Verified that "Next Page" took me to the right place in the result list.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: hskiba

Maniphest Tasks: T13266

Differential Revision: https://secure.phabricator.com/D20455