Fix iterators/compaction issue

[razdoburdin]

Fix wrong search results and unsafe vector reads during concurrent updates and compaction.

1. Hold the search lock until internal IDs are converted to external labels. Mark missing labels as invalid.
2. Track returned labels and clear cached IDs after compaction, so batch iterators avoid duplicates and do not stop too early.
3. Add on_datum and on_data callbacks so callers can copy vectors while locks are held.