Use row comparison in keyset pagination

What does this MR do?

This change uses row comparison in keyset pagination when the ORDER BY columns are not nullable and the order direction is the same.

Thanks, @pbair for suggesting this technique!

Instead of building a complex UNION query we can use: (created_at, id) > ('2012-09-18 01:40:01+00', 15)

• Old query example: https://explain.depesz.com/s/K2Kf
• New query example: https://explain.depesz.com/s/7W79

There is a tiny performance improvement when using the PG row comparison.

The change does not introduce new functionality, the following test cases are covering the change:

'./spec/lib/gitlab/pagination/keyset/order_spec.rb[1:1:2:1:2:1]' # Gitlab::Pagination::Keyset::Order paginate over items correctly when ordering by two non-nullable columns and a distinct column behaves like order examples when paginating forwards
'./spec/lib/gitlab/pagination/keyset/order_spec.rb[1:1:2:1:2:2:1]' # Gitlab::Pagination::Keyset::Order paginate over items correctly when ordering by two non-nullable columns and a distinct column behaves like order examples when paginating forwards with different page size
'./spec/lib/gitlab/pagination/keyset/order_spec.rb[1:1:2:1:3:1]' 

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