RE: POC: enable logical decoding when wal_level = 'replica' without a server restart

I found that after the fix I proposed [1], there is a possibility that effective_wal_level
could be logical after the promotion, even after the logical slots are dropped [2].
Steps:

0. Setup a streaming replication system, and both nodes had a replication slot
1. Attached to the startup and added a break at UpdateLogicalDecodingStatusEndOfRecovery
2. Sent a promotion request to the standby. Startup would stop
3. Established a connection to the standby.
4. Attached to the backend and added a break at ReplicationSlotDrop
5. Tried to drop the replication slot on the standby. Backend would stop
6. Moved the startup till WaitForProcSignalBarrier(). Note that allow_status_change
   was still off.
7. Detached from the backend process.
8. Detached from the startup process.

This can happen because UpdateLogicalDecodingStatusEndOfRecovery() decided to
keep wal_level logical, and upcoming DisableLogicalDecodingIfNecessary() cannot
disable it. allow_status_change should be true for the case.

I considered an approach not to release lock while waiting the ProcSignal, but
other processes cannot not read and update xlog_logical_info.

[1]:
https://www.postgresql.org/message-id/OSCPR01MB14966B8F6F728F3FB4B05BFDBF5E7A%40OSCPR01MB14966.jpnprd01.prod.outlook.com
[2]
```
postgres=# SELECT pg_is_in_recovery();
 pg_is_in_recovery
-------------------
 f
(1 row)

postgres=# SHOW effective_wal_level ;
 effective_wal_level
---------------------
 logical
(1 row)

postgres=# SELECT count(*) FROM pg_replication_slots ;
 count
-------
     0
(1 row)
```

Best regards,
Hayato Kuroda
FUJITSU LIMITED