pgsql: Fix PANIC with track_functions due to concurrent drop of pgstats

Fix PANIC with track_functions due to concurrent drop of pgstats entries

pgstat_drop_entry_internal() generates an ERROR if facing a pgstats
entry already marked as dropped. With a workload doing a lot of
concurrent CALL and DROP/CREATE PROCEDURE, it could be possible for
AtEOXact_PgStat_DroppedStats(), that wants to do transactional drops, to
find entries that are already dropped, after a commit record has been
written. In this case, ERRORs are upgraded to PANIC, taking down the
server.

This issue is fixed by making pgstat_drop_entry() optionally more
tolerant to concurrent drops, adding to the routine a missing_ok option
to make some of its callers more tolerant (spoiler: some of the callers
want a strict behavior, like replication slots and backend stats).
pgstat_drop_entry_internal() cannot be called anymore for an entry
marked as dropped, hence its error is replaced by an assertion.
Functions are handled as a special case in core; this problem could also
apply to custom stats kinds depending on what an extension does.
track_functions is costly when enabled (disabled by default), which is
perhaps the main reason why this has not be found yet.

A similar version of this patch has been proposed by Sami Imseih on a
different thread for a feature in development. This version has tweaked
here by me for the sake of fixing this issue.

Reported-by: zhanglihui <zlh21343(at)163(dot)com>
Author: Sami Imseih <samimseih(at)gmail(dot)com>
Author: Michael Paquier <michael(at)paquier(dot)xyz>
Reviewed-by: Ayush Tiwari <ayushtiwari(dot)slg01(at)gmail(dot)com>
Discussion: https://postgr.es/m/19520-73873648d44793cf@postgresql.org
Backpatch-through: 15

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/850b9218c8e4aa7a56f4ec34a542d4a37f9e07eb

Modified Files
--------------
src/backend/utils/activity/pgstat.c | 2 +-
src/backend/utils/activity/pgstat_function.c | 2 +-
src/backend/utils/activity/pgstat_replslot.c | 2 +-
src/backend/utils/activity/pgstat_shmem.c | 28 +++++++++++++++-------
src/backend/utils/activity/pgstat_xact.c | 8 +++----
src/include/utils/pgstat_internal.h | 3 ++-
.../test_custom_stats/test_custom_var_stats.c | 2 +-
7 files changed, 29 insertions(+), 18 deletions(-)