Refactor FindStripeByRowNumber into StripeMetadataLookupRowNumber
Refactor FindStripeByRowNumber into StripeMetadataLookupRowNumber

Push the most logic in FindStripeByRowNumber down to an helper function
to re-use it in next commit.

citusdata/citus,citusdata/citus,citusdata/citus,citusdata/citus,citusdata/citus