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

Dear Hou,

Thank you for updating the patch! Followings are comments.

1. config.sgml

```
+        the changes till logical_decoding_work_mem is reached. It can also be
```

I think it should be sandwiched by <varname>.

2. config.sgml

```
+        On the publisher side, <varname>logical_replication_mode</varname> allows
+        allows streaming or serializing changes immediately in logical decoding.
```

Typo "allows allows" -> "allows"

3. test general

You confirmed that the leader started to serialize changes, but did not ensure the endpoint.
IIUC the parallel apply worker exits after applying serialized changes, and it is not tested yet.
Can we add polling the log somewhere?


4. 015_stream.pl

```
+is($result, qq(15000), 'all changes are replayed from file')
```

The statement may be unclear because changes can be also replicated when streaming = on.
How about: "parallel apply worker replayed all changes from file"?


Best Regards,
Hayato Kuroda
FUJITSU LIMITED