BUG #14964: statement_timeout cann't set in plpgsql

The following bug has been logged on the website:

Bug reference:      14964
Logged by:          Zhou Digoal
Email address:      digoal@126.com
PostgreSQL version: 10.1
Operating system:   centos 7.4 x64
Description:

```
create or replace function f1(int) returns setof record as $$
declare
begin
  set local statement_timeout='1ms';
  return query select count(*) as cnt, id from a where id<$1 group by id; 
end;
$$ language plpgsql strict ;
```

but it can execute every time, which when the query run more than 1ms.