RE: Perform streaming logical transactions by background workers and parallel apply

Dear Wang,

I found further comments about the test code.

11. src/test/regress/sql/subscription.sql

```
-- fail - streaming must be boolean
CREATE SUBSCRIPTION regress_testsub CONNECTION 'dbname=regress_doesnotexist' PUBLICATION testpub WITH (connect = false, streaming = foo);
```

The comment is no longer correct: should be "streaming must be boolean or 'parallel'"

12. src/test/regress/sql/subscription.sql

```
-- now it works
CREATE SUBSCRIPTION regress_testsub CONNECTION 'dbname=regress_doesnotexist' PUBLICATION testpub WITH (connect = false, streaming = true);
```

I think we should test the case of streaming = 'parallel'.

13. 015_stream.pl

I could not find test about TRUNCATE. IIUC apply bgworker works well
even if it gets LOGICAL_REP_MSG_TRUNCATE message from main worker.
Can you add the case? 

Best Regards,
Hayato Kuroda
FUJITSU LIMITED