Summary:
- Switched all instances of SetMinPossibleForUserKey and SetMaxPossibleForUserKey in accordance to InternalKeyComparator's comparison logic
Closes https://github.com/facebook/rocksdb/pull/2868
Differential Revision: D5804152
Pulled By: axxufb
fbshipit-source-id: 80be35e04f2e8abc35cc64abe1fecb03af24e183