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_4_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/1df4123048c49dda878bbe3b49616b0ff3b3dbe5

Modified Files
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src/backend/access/gist/gistget.c  | 70 +++++++++++++++++++++++++-------------
src/backend/access/gist/gistscan.c | 31 +++++++++++++----
src/include/access/gist_private.h  | 25 ++++++++++++--
3 files changed, 93 insertions(+), 33 deletions(-)