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

On Fri, Jul 7, 2022 at 11:44 AM I wrote:
> Attach the new patches.

I found a failure on CFbot [1], which after investigation I think is due to my
previous modification (see response to #1.10 in [2]).

For a streaming transaction, if we failed in the first chunk of streamed
changes for this transaction in the apply background worker, we will set the
status of this apply background worker to APPLY_BGWORKER_EXIT. 
And at the same time, main apply worker obtains apply background worker
in the function apply_bgworker_find when processing the second chunk of
streamed changes for this transaction, the status of apply background worker
is APPLY_BGWORKER_EXIT. So the following assertion will fail:
```
Assert(status == APPLY_BGWORKER_BUSY);
```

To fix this, before invoking function assert, I try to detect the failure of
apply background worker. If the status is APPLY_BGWORKER_EXIT, then exit with
an error.

I also made some other small improvements.

Attach the new patches.

[1] - https://cirrus-ci.com/task/6383178511286272?logs=test_world#L2636
[2] -
https://www.postgresql.org/message-id/OS3PR01MB62755C6C9A75EB09F7218B589E839%40OS3PR01MB6275.jpnprd01.prod.outlook.com

Regards,
Wang wei