pgsql: Use HTAB for replication slot statistics.

Use HTAB for replication slot statistics.

Previously, we used to use the array of size max_replication_slots to
store stats for replication slots. But that had two problems in the cases
where a message for dropping a slot gets lost: 1) the stats for the new
slot are not recorded if the array is full and 2) writing beyond the end
of the array if the user reduces the max_replication_slots.

This commit uses HTAB for replication slot statistics, resolving both
problems. Now, pgstat_vacuum_stat() search for all the dead replication
slots in stats hashtable and tell the collector to remove them. To avoid
showing the stats for the already-dropped slots, pg_stat_replication_slots
view searches slot stats by the slot name taken from pg_replication_slots.

Also, we send a message for creating a slot at slot creation, initializing
the stats. This reduces the possibility that the stats are accumulated
into the old slot stats when a message for dropping a slot gets lost.

Reported-by: Andres Freund
Author: Sawada Masahiko, test case by Vignesh C
Reviewed-by: Amit Kapila, Vignesh C, Dilip Kumar
Discussion: https://postgr.es/m/20210319185247.ldebgpdaxsowiflw@alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3fa17d37716f978f80dfcdab4e7c73f3a24e7a48

Modified Files
--------------
contrib/test_decoding/t/001_repl_stats.pl | 69 ++++--
src/backend/catalog/system_views.sql | 30 +--
src/backend/postmaster/pgstat.c | 340 +++++++++++++++++-------------
src/backend/replication/logical/logical.c | 2 +-
src/backend/replication/slot.c | 28 +--
src/backend/utils/adt/pgstatfuncs.c | 146 ++++++++-----
src/include/catalog/catversion.h | 2 +-
src/include/catalog/pg_proc.dat | 14 +-
src/include/pgstat.h | 10 +-
src/include/replication/slot.h | 2 +-
src/test/regress/expected/rules.out | 4 +-
src/tools/pgindent/typedefs.list | 2 +-
12 files changed, 389 insertions(+), 260 deletions(-)