pgsql: Allow changing autovacuum_max_workers without restarting.

Allow changing autovacuum_max_workers without restarting.

This commit introduces a new parameter named
autovacuum_worker_slots that controls how many autovacuum worker
slots to reserve during server startup.  Modifying this new
parameter's value does require a server restart, but it should
typically be set to the upper bound of what you might realistically
need to set autovacuum_max_workers.  With that new parameter in
place, autovacuum_max_workers can now be changed with a SIGHUP
(e.g., pg_ctl reload).

If autovacuum_max_workers is set higher than
autovacuum_worker_slots, a WARNING is emitted, and the server will
only start up to autovacuum_worker_slots workers at a given time.
If autovacuum_max_workers is set to a value less than the number of
currently-running autovacuum workers, the existing workers will
continue running, but no new workers will be started until the
number of running autovacuum workers drops below
autovacuum_max_workers.

Reviewed-by: Sami Imseih, Justin Pryzby, Robert Haas, Andres Freund, Yogesh Sharma
Discussion: https://postgr.es/m/20240410212344.GA1824549%40nathanxps13

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c758119e5bfb47b38cf957f9a5a37ceae96fa9b3

Modified Files
--------------
doc/src/sgml/config.sgml                      | 28 ++++++++-
doc/src/sgml/runtime.sgml                     |  4 +-
src/backend/access/transam/xlog.c             |  2 +-
src/backend/postmaster/autovacuum.c           | 82 +++++++++++++++++++++------
src/backend/postmaster/pmchild.c              |  4 +-
src/backend/storage/lmgr/proc.c               |  6 +-
src/backend/utils/init/postinit.c             |  6 +-
src/backend/utils/misc/guc_tables.c           | 11 +++-
src/backend/utils/misc/postgresql.conf.sample |  3 +-
src/include/postmaster/autovacuum.h           |  1 +
src/test/perl/PostgreSQL/Test/Cluster.pm      |  1 +
11 files changed, 117 insertions(+), 31 deletions(-)