pgsql: Add INJECTION_POINT_CACHED() to run injection points directly fr

Add INJECTION_POINT_CACHED() to run injection points directly from cache

This new macro is able to perform a direct lookup from the local cache
of injection points (refreshed each time a point is loaded or run),
without touching the shared memory state of injection points at all.

This works in combination with INJECTION_POINT_LOAD(), and it is better
than INJECTION_POINT() in a critical section due to the fact that it
would avoid all memory allocations should a concurrent detach happen
since a LOAD(), as it retrieves a callback from the backend-private
memory.

The documentation is updated to describe in more details how to use this
new macro with a load.  Some tests are added to the module
injection_points based on a new SQL function that acts as a wrapper of
INJECTION_POINT_CACHED().

Based on a suggestion from Heikki Linnakangas.

Author: Heikki Linnakangas, Michael Paquier
Discussion: https://postgr.es/m/58d588d0-e63f-432f-9181-bed29313dece@iki.fi

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/a0a5869a8598cdeae1d2f2d632038d26dcc69d19

Modified Files
--------------
doc/src/sgml/xfunc.sgml                                 | 17 ++++++++++-------
src/backend/utils/misc/injection_point.c                | 17 +++++++++++++++++
src/include/utils/injection_point.h                     |  3 +++
.../injection_points/expected/injection_points.out      | 13 +++++++++++++
.../modules/injection_points/injection_points--1.0.sql  | 10 ++++++++++
src/test/modules/injection_points/injection_points.c    | 14 ++++++++++++++
.../modules/injection_points/sql/injection_points.sql   |  2 ++
7 files changed, 69 insertions(+), 7 deletions(-)