[PATCH] Don't block HOT update by BRIN index

Hello!

Tomáš Vondra has shared a few ideas to improve BRIN index in czech
PostgreSQL mail list some time ago [1 , in czech only]. This is first
try to implement one of those ideas.

Currently BRIN index blocks HOT update even it is not linked tuples
directly. I'm attaching the initial patch allowing HOT update even on
BRIN indexed columns. This patch went through an initial review on
czech PostgreSQL mail list [1].

It can be viewed online (latest version) on GitHub [2] as well.

- small overview

1. I have added "amhotblocking" flag to index AM descriptor set to
"true" for all, except BRIN, index types. And later in heap_update
method (heapam.c) I do filter attributes based on this new flag,
instead of currently checking for any existing index.

2. I had to enhance the "RelationGetIndexAttrBitmap" function to be
able to return a bitmap of index attribute numbers related to the new
AM flag using "INDEX_ATTR_BITMAP_HOT_BLOCKING" filter.
PS: Originally the "INDEX_ATTR_BITMAP_ALL" filter was used for HOT
check update and most likely could be removed (including all logic
related in RelationGetIndexAttrBitmap), since I have not found any
other usage.

3. I have created an initial regression test using
"pg_stat_get_tuples_hot_updated" to find out HOT was successful on the
BRIN indexed column. Unfortunately "pg_stat_get_tuples_hot_updated" is
not updated immediately and I have not found any way to enforce the
update. Thus (at least for now) I have used a similar approach to
stats.sql using the "wait_for_stats" function (waiting for 30 seconds
and checking each 100ms for change).

I'm attaching patch to CF 2021-07.

[1] https://groups.google.com/g/postgresql-cz/c/oxA_v3H17Qg
[2] https://github.com/simi/postgres/pull/7