BUG #15095: schema's owner can drop object's in his schema

The following bug has been logged on the website:

Bug reference:      15095
Logged by:          Zhou Digoal
Email address:      digoal@126.com
PostgreSQL version: 10.2
Operating system:   centos 7.x x64
Description:

Schema's owner can drop object's in his schema, I think this is an risk if
multi-user use the same schema. 

exp:

```
postgres=# create database pp owner postgres;
CREATE DATABASE
postgres=# grant create on database pp to pp;
GRANT
postgres=# \c pp pp
You are now connected to database "pp" as user "pp".
pp=> create schema abc;
CREATE SCHEMA
pp=> \c pp postgres
You are now connected to database "pp" as user "postgres".
pp=# create table abc.a(id int);
CREATE TABLE
pp=# insert into abc.a values (1);
INSERT 0 1
pp=# \c pp pp
You are now connected to database "pp" as user "pp".
pp=> select * from abc.a;
ERROR:  permission denied for relation a
pp=> drop table abc.a;
DROP TABLE
```

best regards,
digoal.