Fix a bug in db_stress and an incorrect assertion in FilePickerMultiGet (#5301)

Fix a bug in db_stress and an incorrect assertion in FilePickerMultiGet (#5301)

Summary:
This PR has two fixes for crash test failures -
1. Fix a bug in TestMultiGet() in db_stress that was passing list of key to MultiGet() in the wrong order, thus ensuring that actual values don't match expected values
2. Remove an incorrect assertion in FilePickerMultiGet::GetNextFileInLevelWithKeys() that checks that files in a level are in sorted order. This is not true with MultiGet(), especially if there are duplicate keys and we may have to go back one file for the next key. Furthermore, this assertion makes more sense when a new version is created, rather than at lookup time

Test -
asan_crash and ubsan_crash tests
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5301

Differential Revision: D15337383

Pulled By: anand1976

fbshipit-source-id: 35092cb15bbc1700e5e823cbe07bfa62f1e9e6c6