Disallow server start with sync_replication_slots = on and wal_level < logical.

Replication slot synchronization (sync_replication_slots = on)
requires wal_level to be logical. This commit prevents the server
from starting if sync_replication_slots is enabled but wal_level
is set to minimal or replica.

Failing early during startup helps users catch invalid configurations
immediately, which is important because changing wal_level requires
a server restart.

Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Shveta Malik <shveta.malik@gmail.com>
Discussion: https://postgr.es/m/CAH0PTU_pc3oHi__XESF9ZigCyzai1Mo3LsOdFyQA4aUDkm01RA@mail.gmail.com

src/backend/postmaster/postmaster.c

@@ -854,6 +854,9 @@ PostmasterMain(int argc, char *argv[])
 	if (summarize_wal && wal_level == WAL_LEVEL_MINIMAL)
 		ereport(ERROR,
 				(errmsg("WAL cannot be summarized when \"wal_level\" is \"minimal\"")));
+	if (sync_replication_slots && wal_level < WAL_LEVEL_LOGICAL)
+		ereport(ERROR,
+				(errmsg("replication slot synchronization (\"sync_replication_slots\" = on) requires \"wal_level\" >= \"logical\"")));
 
 	/*
 	 * Other one-time internal sanity checks can go here, if they are fast.