[Pagination]Improve ORDER BY index usage

See https://forums.couchbase.com/t/n1ql-select-vs-select-lots-of-fieldnames-performance/9406/11

Two improvements needed.

(1) Efficient support for DESC in ORDER BY, using index order.

(2) Leaving out the leading index keys in ORDER BY, if those leading keys have fixed / constant values based on the query WHERE clause.