Fix flush picking non-consecutive memtables (#10921)

Fix flush picking non-consecutive memtables (#10921)

Summary:
Prevents `MemTableList::PickMemtablesToFlush()` from picking non-consecutive memtables. It leads to wrong ordering in L0 if the files are committed, or an error like below if force_consistency_checks=true catches it:

```
Corruption: force_consistency_checks: VersionBuilder: L0 file https://github.com/facebook/rocksdb/issues/25 with seqno 320416 368066 vs. file https://github.com/facebook/rocksdb/issues/24 with seqno 336037 352068
```

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10921

Test Plan: fix the expectation in the existing test of this behavior

Reviewed By: riversand963

Differential Revision: D41046935

Pulled By: ajkr

fbshipit-source-id: 783696bff56115063d5dc5856dfaed6a9881d1ab