Change commit_delay to be SUSET for 9.3+

Prior to 9.3 the commit_delay affected only the current user,
whereas now only the group leader waits while holding the
WALWriteLock. Deliberate or accidental settings to a poor
value could seriously degrade performance for all users.
Privileges may be delegated by SECURITY DEFINER functions
for anyone that needs per-user settings in real situations.
Request for change from Peter Geoghegan

Author: simonat2ndQuadrant

src/backend/utils/misc/guc.c

@@ -2042,7 +2042,7 @@ static struct config_int ConfigureNamesInt[] =
 	},
 
 	{
-		{"commit_delay", PGC_USERSET, WAL_SETTINGS,
+		{"commit_delay", PGC_SUSET, WAL_SETTINGS,
 			gettext_noop("Sets the delay in microseconds between transaction commit and "
 						 "flushing WAL to disk."),
 			NULL