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

src/test/perl/PostgreSQL/Test/Cluster.pm

@@ -707,6 +707,7 @@ sub init
 		}
 		print $conf "max_wal_senders = 10\n";
 		print $conf "max_replication_slots = 10\n";
+		print $conf "autovacuum_worker_slots = 3\n";
 		print $conf "wal_log_hints = on\n";
 		print $conf "hot_standby = on\n";
 		# conservative settings to ensure we can run multiple postmasters: