Reduce SortExec memory usage by void constructing single huge batch (#…

…2132)

* wip

* workable version

* sort to bench

* use sort2

* Replace

* revert unexpected changes

* iter rework

* minor

* prepare

* avoid evaluating sort columns twice

* revert unrelated changes

* Update datafusion/core/src/physical_plan/sorts/sort.rs

Co-authored-by: Andrew Lamb <andrew@nerdnetworks.org>

* resolve comments

* refactor a little bit

* assertion

Co-authored-by: Andrew Lamb <andrew@nerdnetworks.org>