Improve handling of vertices with large cardinality

If a vertex has millions of values on a property, then schema sync becomes unstable.

## Related issues

- Original issue #1050

## Potential solution

- Detect large cardinality in the summary report from Neptune
- Fallback to non-batched queries to get samples of vertices, marking individual vertex labels as problematic

No files or tests are named. Start by tracing schema sync and the Neptune summary report, then determine how vertex sampling is performed. Done means detecting large cardinality, falling back to non-batched sampling, and marking affected vertex labels as problematic.

Written by the indexing model from the issue text.