store: Fix a mistake in how pruning status is reported

[lutter]

When we start copying nonfinal entities, we need to reset the start_vid and next_vid. They don't necessarily have a direct connection to the same values for nonfinal entities; e.g., it is possible that the start_vid for nonfinal entities is smaller than the one for final entities.

This change only affects how we report the completion of prune operations; it has no effect on what gets pruned and how.