Add max_sync_workers_per_subscription to postgresql.conf.sample.

This commit also does

- add REPLICATION_SUBSCRIBERS into config_group
- mark max_logical_replication_workers and max_sync_workers_per_subscription
  as REPLICATION_SUBSCRIBERS parameters
- move those parameters into "Subscribers" section in postgresql.conf.sample

Author: Masahiko Sawada, Petr Jelinek and me
Reported-by: Masahiko Sawada
Discussion: http://postgr.es/m/CAD21AoAonSCoa=v=87ZO3vhfUZA1k_E2XRNHTt=xioWGUa+0ug@mail.gmail.com

src/backend/utils/misc/guc.c

@@ -610,6 +610,8 @@ const char *const config_group_names[] =
 	gettext_noop("Replication / Master Server"),
 	/* REPLICATION_STANDBY */
 	gettext_noop("Replication / Standby Servers"),
+	/* REPLICATION_SUBSCRIBERS */
+	gettext_noop("Replication / Subscribers"),
 	/* QUERY_TUNING */
 	gettext_noop("Query Tuning"),
 	/* QUERY_TUNING_METHOD */
@@ -2511,7 +2513,7 @@ static struct config_int ConfigureNamesInt[] =
 	{
 		{"max_logical_replication_workers",
 			PGC_POSTMASTER,
-			RESOURCES_ASYNCHRONOUS,
+			REPLICATION_SUBSCRIBERS,
 			gettext_noop("Maximum number of logical replication worker processes."),
 			NULL,
 		},
@@ -2523,7 +2525,7 @@ static struct config_int ConfigureNamesInt[] =
 	{
 		{"max_sync_workers_per_subscription",
 			PGC_SIGHUP,
-			RESOURCES_ASYNCHRONOUS,
+			REPLICATION_SUBSCRIBERS,
 			gettext_noop("Maximum number of table synchronization workers per subscription."),
 			NULL,
 		},

src/backend/utils/misc/postgresql.conf.sample

@@ -164,7 +164,6 @@
 #max_parallel_workers_per_gather = 2	# taken from max_parallel_workers
 #max_parallel_workers = 8	    # maximum number of max_worker_processes that
 					# can be used in parallel queries
-#max_logical_replication_workers = 4	# taken from max_worker_processes
 #old_snapshot_threshold = -1		# 1min-60d; -1 disables; 0 is immediate
 					# (change requires restart)
 #backend_flush_after = 0		# measured in pages, 0 disables
@@ -273,6 +272,13 @@
 #wal_retrieve_retry_interval = 5s	# time to wait before retrying to
 					# retrieve WAL after a failed attempt
 
+# - Subscribers -
+
+# These settings are ignored on a publisher.
+
+#max_logical_replication_workers = 4	# taken from max_worker_processes
+#max_sync_workers_per_subscription = 2	# taken from max_logical_replication_workers
+
 
 #------------------------------------------------------------------------------
 # QUERY TUNING

src/include/utils/guc_tables.h

@@ -72,6 +72,7 @@ enum config_group
 	REPLICATION_SENDING,
 	REPLICATION_MASTER,
 	REPLICATION_STANDBY,
+	REPLICATION_SUBSCRIBERS,
 	QUERY_TUNING,
 	QUERY_TUNING_METHOD,
 	QUERY_TUNING_COST,