Summary:
We are seeing a bug of wrong results with merging iterator's reseek avoidence feature and prefix extractor. Disable this optimization for now.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5815

Test Plan: Validated the same MyRocks case was fixed; run all existing tests.

Differential Revision: D17430776

fbshipit-source-id: aef664277ba0ab8a2e68331ff0db6ae682535371