Stale reads when using `Iterator::Refresh()`

### Expected behavior

The expected behavior is that reads through an iterator should see all writes that have completed prior to calling `Refresh()`.

### Actual behavior

Reads through such an iterator sometimes miss updates that were committed prior to calling `Refresh()`

### Steps to reproduce the behavior

Unfortunately, we don't have a great reproduction outside of the FoundationDB integration currently. We have been trying to replicate outside of FoundationDB and would be more than happy to help anyone working on this build FDB and run the tests that expose this bug.