Implement collect_block for lazy scorers.

[stuhood]

The new API proposed in #2726 enables implementing Collector::collect_block, which allows for slightly more efficient bulk pushes to the TopNComputer when a SegmentSortKeyComputer::is_lazy, and when scores are not required.

We can:

• Check capacity and truncate once for the entire block.
• Lift the check for the presence of a threshold out of the loop over the block.

It would additionally allow for bulk fetching (using ColumnValues::first_vals) the first sort key column, as future work.

I haven't benchmarked this yet! But collect_block is quite beneficial for aggregations.