pgsql: nbtree: Make BTMaxItemSize into object-like macro.

nbtree: Make BTMaxItemSize into object-like macro.

Make nbtree's "1/3 of a page limit" BTMaxItemSize function-like macro
(which accepts a "page" argument) into an object-like macro that can be
used from code that doesn't have convenient access to an nbtree page.

Preparation for an upcoming patch that adds skip scan to nbtree.
Parallel index scans that use skip scan will serialize datums (not just
SAOP array subscripts) when scheduling primitive scans. BTMaxItemSize
will be used by btestimateparallelscan to determine how much DSM to
request.

Author: Peter Geoghegan <pg(at)bowt(dot)ie>
Discussion: https://postgr.es/m/CAH2-Wz=H_RG5weNGeUG_TkK87tRBnH9mGCQj6WpM4V4FNWKv2g@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/426ea611171da4e60ab4f3863fa3cc3683ae9547

Modified Files
--------------
contrib/amcheck/verify_nbtree.c | 3 +--
src/backend/access/nbtree/nbtdedup.c | 6 +++---
src/backend/access/nbtree/nbtinsert.c | 2 +-
src/backend/access/nbtree/nbtsort.c | 4 ++--
src/backend/access/nbtree/nbtutils.c | 7 +++----
src/backend/access/nbtree/nbtxlog.c | 2 +-
src/include/access/nbtree.h | 8 ++++----
7 files changed, 15 insertions(+), 17 deletions(-)