Currently, KNN searches were supported only by GiST.  SP-GiST also capable to
support them.  This commit implements that support.  SP-GiST scan stack is
replaced with queue, which serves as stack if no ordering is specified.  KNN
support is provided for three SP-GIST opclasses: quad_point_ops, kd_point_ops
and poly_ops (catversion is bumped).  Some common parts between GiST and SP-GiST
KNNs are extracted into separate functions.

Discussion: https://postgr.es/m/570825e8-47d0-4732-2bf6-88d67d2d51c8%40postgrespro.ru
Author: Nikita Glukhov, Alexander Korotkov based on GSoC work by Vlad Sterzhanov
Review: Andrey Borodin, Alexander Korotkov