Octavia does not handle DBConnection error on batch_update_members

Octavia handles the code for DBConnectionError in API and during execution taskflow.
However such error is not handled during preparation tasks here:
https://github.com/openstack/octavia/blob/stable/2023.1/octavia/controller/worker/v1/controller_worker.py#L496-L516

On my setup with octavia yoga, I get DBConnectionError during getting list of old members.

```
2023-03-15 13:28:17,652.652 10 ERROR oslo_messaging.rpc.server self.connect()
2023-03-15 13:28:17,652.652 10 ERROR oslo_messaging.rpc.server File "/var/lib/openstack/lib/python3.8/site-packages/pymysql/connections.py", line 664, in connect
2023-03-15 13:28:17,652.652 10 ERROR oslo_messaging.rpc.server raise exc
2023-03-15 13:28:17,652.652 10 ERROR oslo_messaging.rpc.server oslo_db.exception.DBConnectionError: (pymysql.err.OperationalError) (2003, "Can't connect to MySQL server on 'x.x.x.x' ([Errno 111] Connection refused)")
2023-03-15 13:28:17,652.652 10 ERROR oslo_messaging.rpc.server [SQL: SELECT 1]
2023-03-15 13:28:17,652.652 10 ERROR oslo_messaging.rpc.server (Background on this error at: https://sqlalche.me/e/14/e3q8)
2023-03-15 13:28:17,652.652 10 ERROR oslo_messaging.rpc.server
```

Another trace:

https://paste.opendev.org/show/btK99zWWfozmXCizn58L/

```
2023-04-03 10:22:09,964.964 10 ERROR oslo_messaging.rpc.server File "/var/lib/openstack/lib/python3.8/site-packages/octavia/controller/worker/v1/controller_worker.py", line 502, in batch_update_members
2023-04-03 10:22:09,964.964 10 ERROR oslo_messaging.rpc.server updated_members = [
2023-04-03 10:22:09,964.964 10 ERROR oslo_messaging.rpc.server File "/var/lib/openstack/lib/python3.8/site-packages/octavia/controller/worker/v1/controller_worker.py", line 503, in <listcomp>
2023-04-03 10:22:09,964.964 10 ERROR oslo_messaging.rpc.server (self._member_repo.get(db_apis.get_session(), id=m.get('id')), m)
2023-04-03 10:22:09,964.964 10 ERROR oslo_messaging.rpc.server File "/var/lib/openstack/lib/python3.8/site-packages/octavia/db/repositories.py", line 139, in get
2023-04-03 10:22:09,964.964 10 ERROR oslo_messaging.rpc.server return model.to_data_model()

...

2023-04-03 10:22:09,964.964 10 ERROR oslo_messaging.rpc.server raise err.OperationalError(
2023-04-03 10:22:09,964.964 10 ERROR oslo_messaging.rpc.server oslo_db.exception.DBConnectionError: (pymysql.err.OperationalError) (2013, 'Lost connection to MySQL server during query')
2023-04-03 10:22:09,964.964 10 ERROR oslo_messaging.rpc.server [SQL: SELECT tags.resource_id AS tags_resource_id, tags.tag AS tags_tag, anon_1.pool_id AS anon
_1_pool_id
2023-04-03 10:22:09,964.964 10 ERROR oslo_messaging.rpc.server FROM (SELECT pool.id AS pool_id
2023-04-03 10:22:09,964.964 10 ERROR oslo_messaging.rpc.server FROM pool
2023-04-03 10:22:09,964.964 10 ERROR oslo_messaging.rpc.server WHERE pool.id = %(pk_1)s) AS anon_1 INNER JOIN tags ON tags.resource_id = anon_1.pool_id]
2023-04-03 10:22:09,964.964 10 ERROR oslo_messaging.rpc.server [parameters: {'pk_1': 'da2108ed-cb40-4d8d-a0f2-0ab3468c4b8c'}]
2023-04-03 10:22:09,964.964 10 ERROR oslo_messaging.rpc.server (Background on this error at: https://sqlalche.me/e/14/e3q8)
2023-04-03 10:22:09,964.964 10 ERROR oslo_messaging.rpc.server
```

As result both LBs stuck in PENDING_UPDATE state.