Merging should pass correct fieldinfos to producers always [LUCENE-5990]

I think its been a longstanding issue, but I noticed it in the bulk merge code today and see that we can fix it...

instead of:

```Java
DocumentStoredFieldVisitor visitor = new DocumentStoredFieldVisitor();
storedFieldsReader.visitDocument(docID, visitor);
Document doc = visitor.getDocument();
addDocument(doc, mergeState.mergeFieldInfos);
```

we should do:

```Java
addDocument(doc, mergeState.fieldInfos[i]);
```

This is a lot more consistent and reduce the possibility of scary bugs during merge because the codec does something strange. We should look into all merge logic to see if it can be improved here.

---
Migrated from [LUCENE-5990](https://issues.apache.org/jira/browse/LUCENE-5990) by Robert Muir (@rmuir)

Start with the bulk merge code around DocumentStoredFieldVisitor, storedFieldsReader.visitDocument, addDocument, and mergeState. Inspect the other merge paths for the same fieldInfos handling; done means producers consistently receive the per-segment fieldInfos rather than the shared mergeFieldInfos.

Written by the indexing model from the issue text.