RE: Make default subscription streaming option as Parallel

Dear Amit, Vignesh,

> 1. Please ensure that none of the existing tests that use
> subscriptions with large changes will be impacted due to this change.

I found at least 022_twophase_cascade.pl should be fixed.
The file has a part which tests non-streaming case:

```
# -----------------------
# 2PC NON-STREAMING TESTS
# -----------------------
...
$node_B->safe_psql(
    'postgres', "
    CREATE SUBSCRIPTION tap_sub_B
    CONNECTION '$node_A_connstr application_name=$appname_B'
    PUBLICATION tap_pub_A
    WITH (two_phase = on)");
...
```

I know the streaming actually does not happen because few tuples will be inserted
later, but creating as streaming=parallel is bit misleading.

I checked other files as well but I couldn't find what we should fix.

> 2. The pg_createsubscriber utility uses CREATE SUBSCRIPTION statement
> and after this change, it will enable parallel mode by default which I
> think is a good idea as users won't need to do that manually after
> running the tool. Do you see any problem with this?

I also think it is okay. IIUC, there were no specific reasons to create
subscriptions with streaming=off, it was chosen because it was a default.
I cannot find strong reasons to keep current behavior.

Best regards,
Hayato Kuroda
FUJITSU LIMITED