pgsql: Use a pairing heap for the priority queue in kNN-GiST searches.

Use a pairing heap for the priority queue in kNN-GiST searches.

This performs slightly better, uses less memory, and needs slightly less
code in GiST, than the Red-Black tree previously used.

Reviewed by Peter Geoghegan

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/e7032610f76e6c8345496ae7bbdf49d3c40df30f

Modified Files
--------------
src/backend/access/gist/gistget.c  |   71 +++-------
src/backend/access/gist/gistscan.c |   75 ++--------
src/backend/lib/Makefile           |    2 +-
src/backend/lib/README             |   24 ++++
src/backend/lib/pairingheap.c      |  274 ++++++++++++++++++++++++++++++++++++
src/include/access/gist_private.h  |   25 +---
src/include/lib/pairingheap.h      |   72 ++++++++++
7 files changed, 409 insertions(+), 134 deletions(-)