Initialize work_mem using current guc.c default.

Do the same for the maintenance_work_mem global variable.

Oversight in commit 848ae33, which increased the previous defaults
for work_mem and maintenance_work_mem by 4X.

Author: petergeoghegan

src/backend/utils/init/globals.c

@@ -118,8 +118,8 @@ int			IntervalStyle = INTSTYLE_POSTGRES;
 
 bool		enableFsync = true;
 bool		allowSystemTableMods = false;
-int			work_mem = 1024;
-int			maintenance_work_mem = 16384;
+int			work_mem = 4096;
+int			maintenance_work_mem = 65536;
 int			max_parallel_maintenance_workers = 2;
 
 /*