[PATCH] Incremental sort (was: PoC: Partial sort)

Hi all!

I decided to start new thread for this patch for following two reasons.
* It's renamed from "Partial sort" to "Incremental sort" per suggestion by
Robert Haas [1]. New name much better characterizes the essence of
algorithm.
* I think it's not PoC anymore. Patch received several rounds of review
and now it's in the pretty good shape.

Attached revision of patch has following changes.
* According to review [1], two new path and plan nodes are responsible for
incremental sort: IncSortPath and IncSort which are inherited from SortPath
and Sort correspondingly. That allowed to get rid of set of hacks with
minimal code changes.
* According to review [1] and comment [2], previous tuple is stored in
standalone tuple slot of SortState rather than just HeapTuple.
* New GUC parameter enable_incsort is introduced to control planner
ability to choose incremental sort.
* Test of postgres_fdw with not pushed down cross join is corrected. It
appeared that with incremental sort such query is profitable to push down.
I changed ORDER BY columns so that index couldn't be used. I think this
solution is more elegant than setting enable_incsort = off.

Also patch has set of assorted code and comments improvements.

Links
1.
https://www.postgresql.org/message-id/CA+TgmoZapyHRm7NVyuyZ+yAV=U1a070BOgRe7PkgyrAegR4JDA@mail.gmail.com
2.
https://www.postgresql.org/message-id/CAM3SWZQL4yD2SnDheMCGL0Q2b2oTdKUvv_L6Zg_FcGoLuwMffg@mail.gmail.com

------
Alexander Korotkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company