msg: fix deadlock when handling existing but closed v2 connection

msg: fix deadlock when handling existing but closed v2 connection

The deadlock is illustrated best by the following snippet
provided by jianwei zhang who also made the problem analysis
(many thanks!).

```
thread-35
AsyncMessenger::shutdown_connections         hold             AsyncMessenger::lock            std::lock_guard l{lock}
AsyncConnection::stop                         wait                AsyncConnection::lock            lock.lock()

thread-3
ProtocolV2::handle_existing_connection        hold                AsyncConnection::lock            std::lock_guard<std::mutex> l(existing->lock)
AsyncMessenger::accept_conn                wait                AsyncMessenger::lock            std::lock_guard l{lock}
```

Fixes: https://tracker.ceph.com/issues/55355
Signed-off-by: Radosław Zarzyński <rzarzyns@redhat.com>