typeORMのissue下書き

`getMany` get reduced rows from tables using timezone primary key

**Issue type:**

[ ] question
[x] bug report
[ ] feature request
[ ] documentation issue

**Database system/driver:**

[ ] `cordova`
[ ] `mongodb`
[ ] `mssql`
[ ] `mysql` / `mariadb`
[ ] `oracle`
[x] `postgres`
[ ] `cockroachdb`
[ ] `sqlite`
[ ] `sqljs`
[ ] `react-native`
[ ] `expo`

**TypeORM version:**

[ ] `latest`
[ ] `@next`
[ ] `0.x.x` (or put your version here)

**Steps to reproduce or a small repository showing the problem:**


***entity:***

```
@Entity('some_table')
export class SomeTable {
  @PrimaryColumn('text', { name: 'string_key' })
  stringKey!: string;

  @PrimaryColumn('timestamptz', { name: 'timestamptz_key' })
  timestamptzKey!: Date;

  @Column('text', { name: 'content' })
  content!: string;
}
```

***call getMany:***

```
export class CustomRepository {
  constructor(private manager: EntityManager) {}

  async get(): Promise<SomeTable[]> {
    const eventEntities = await this.manager
      .getRepository(SomeTable)
      .createQueryBuilder('some_table')
      .getMany();
   }
}
```

***data:***

|string_key	|timestamptz_key|	content|
|-----------|---------------|---------
|aaa	|2020-05-18 07:42:29.392000 +00:00	|some1|
|aaa	|2020-05-18 07:42:29.678000 +00:00	|some2|


***result:***

only return one row.

***why:***

https://github.com/typeorm/typeorm/blob/d7256c710ec6b4fdecdced002d77eb03417fbc96/src/query-builder/transformer/RawSqlResultsToEntityTransformer.ts#L65

In this section, grouping rows via join primary key value as string.
If column is `Timestamp` type, toString method (which called Implicitly by `join("_")`) truncate omit milli second.

like this.

```
const now = new Date();
console.log(now);
Tue May 19 2020 12:31:20 GMT+0900
```

67: maybe better to use different key?
72: I think better to put this row, that returned
79: truncate millisecondsかな

other looks good