pgsql: Fix handling of NULL distances in KNN-GiST

Fix handling of NULL distances in KNN-GiST

In order to implement NULL LAST semantic GiST previously assumed distance to
the NULL value to be Inf. However, our distance functions can return Inf and
NaN for non-null values. In such cases, NULL LAST semantic appears to be
broken. This commit fixes that by introducing separate array of null flags for
distances.

Backpatch to all supported versions.

Discussion: https://postgr.es/m/CAPpHfdsNvNdA0DBS%2BwMpFrgwT6C3-q50sFVGLSiuWnV3FqOJuQ%40mail.gmail.com
Author: Alexander Korotkov
Backpatch-through: 9.4

Branch
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REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/3c155bafa59b26e0bb38b8e70b9034183c946d39

Modified Files
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src/backend/access/gist/gistget.c | 77 +++++++++++++++++++++++++-------------
src/backend/access/gist/gistscan.c | 25 +++++++++++--
src/include/access/gist_private.h | 26 +++++++++++--
3 files changed, 95 insertions(+), 33 deletions(-)