Avoid some obj allocations when table stats only updates its count and modify count

## Enhancement

We'll call `statistics.Table::Copy` even if we only update its count/modify_count. The `Copy` will do deep clone of each object inside the `Table`.

This can be improved to reduce the pressure on golang's memory control.

Start by locating statistics.Table::Copy and the code path that updates only count and modify_count. Trace how Copy deep-clones the table objects, then determine the existing behavior that must remain unchanged while reducing allocations; validate the result with the relevant statistics tests.

Written by the indexing model from the issue text.