Summary:
When doing a Seek() or SeekForPrev() we should stop the moment we see a key with a different prefix as start if ReadOptions:: prefix_same_as_start was set to true

Right now we don't stop if we encounter a tombstone outside the prefix while executing SeekForPrev()
Closes https://github.com/facebook/rocksdb/pull/3067

Differential Revision: D6149638

Pulled By: IslamAbdelRahman

fbshipit-source-id: 7f659862d2bf552d3c9104a360c79439ceba2f18