pg_createsubscriber: drop pre-existing subscriptions from the converted node

Dear Hackers,

This is a follow-up thread for pg_createsubscriber [1]. I started a new thread
since there is no activity around here.

## Problem

Assuming that there is a cascading replication like below:

node A --(logical replication)--> node B --(streaming replication)--> node C

In this case, subscriptions exist even on node C, but it does not try to connect
to node A because the logical replication launcher/worker won't be launched.
After the conversion, node C becomes a subscriber for node B, and the subscription
toward node A remains. Therefore, another worker that tries to connect to node A
will be launched, raising an ERROR [2]. This failure may occur even during the
conversion.

## Solution

The easiest solution is to drop pre-existing subscriptions from the converted node.
To avoid establishing connections during the conversion, slot_name is set to NONE
on the primary first, then drop on the standby. The setting will be restored on the
primary node.
Attached patch implements the idea. Test script is also included, but not sure it should
be on the HEAD

BTW, I found that LogicalRepInfo.oid won't be used. If needed, I can create
another patch to remove the attribute.

How do you think?

[1]: https://www.postgresql.org/message-id/CAA4eK1J22UEfrqx222h5j9DQ7nxGrTbAa_BC%2B%3DmQXdXs-RCsew%40mail.gmail.com
[2]: https://www.postgresql.org/message-id/CANhcyEWvimA1-f6hSrA%3D9qkfR5SonFb56b36M%2B%2BvT%3DLiFj%3D76g%40mail.gmail.com

Best Regards,
Hayato Kuroda
FUJITSU LIMITED
https://www.fujitsu.com/