Summary: #7124486: RocksDB's Iterator.SeekToLast should seek to the last key before iterate_upper_bound if presents

Test Plan: ./db_iter_test run successfully with the new testcase

Reviewers: rven, yhchiang, igor, anthony, kradhakrishnan, sdong

Reviewed By: sdong

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D40425