Fix pagination of fulltext search results

Summary:
Fixes T8285. Fulltext search relies on an underlying engine which can not realistically use cursor paging. This is unusual and creates some oddness.

Tweak a few numbers -- and how offsets are handled -- to separate the filtered offset and unfiltered offset.

Test Plan:

• Set page size to 2.
• Ran a query.
• Paged forward and backward through results sensibly, seeing the full result set.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8285

Differential Revision: https://secure.phabricator.com/D17667