race condition in OptimisticTransaction::CommitWithParallelValidate

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### Expected behavior
OptimisticTransaction::CommitWithParallelValidate() actually prevent race between TransactionUtil::CheckKeysForConflicts() and DBImpl::Write(), as implied by [its example code](https://github.com/facebook/rocksdb/blob/v7.5.3/examples/optimistic_transaction_example.cc#L117)

### Actual behavior
OptimisticTransaction::CommitWithParallelValidate() cannot prevect race between TransactionUtil::CheckKeysForConflicts() and DBImpl::Write()

### Steps to reproduce the behavior
Read the source code, sepcifically between [calling TransactionUtil::CheckKeysForConflicts](https://github.com/facebook/rocksdb/blob/v7.5.3/utilities/transactions/optimistic_transaction.cc#L123) and [calling DBImpl::Write()](https://github.com/facebook/rocksdb/blob/v7.5.3/utilities/transactions/optimistic_transaction.cc#L129)

The moment thread T1 has just done executing L123, another thread T2 write something into db, then T1 executes L129, the transaction will falsely commit, contrary to what is implied by the [occ example code](https://github.com/facebook/rocksdb/blob/v7.5.3/examples/optimistic_transaction_example.cc#L117)