doc_count_error_upper_bound is higher than necessary

[PSeitz]

After merging #6674, doc_count_error_upper_bound is higher than necessary

[trinity-1686a]

we can recoup that by only running intermediate pruning on leaf nodes (i.e. tell the QuickwitCollector whether it's merging for a root or leaf node, and not run the pruning on root. Final pruning will happen through finalization anyway)