PublishAsync fails with ObjectDisposedException on 'AsyncLockWaiter'

### Verification

### Describe the bug
Running an application with an MQTT client connected to a VerneMQ broker running as a docker container on a different host.
The client is publishing successfully at a rate of 92 msg/s.
When the vernemq container is paused to simulate a communication failure, each PublishAsync operation times out due to the corresponding CancellationToken (created with the designated timeout time).
client.DisconnectedAsync handler is never called also after waiting for KeepAlivePeriod to be elapsed.

After the vernemq container is resumed, some PublishAsync that may be still pending completed with the following exception:
```batch
MQTTTransportDataSource...Publish(messageid:bd169c58-e910-4c63-bdec-8e7743f2f9e0)..Communication error with the broker: Cannot access a disposed object.
Object name: 'AsyncLockWaiter'.
System.ObjectDisposedException: Cannot access a disposed object.
Object name: 'AsyncLockWaiter'.
at MQTTnet.Adapter.MqttChannelAdapter.SendPacketAsync(MqttPacket packet, CancellationToken cancellationToken)
at MQTTnet.Client.MqttClient.PublishAtMostOnce(MqttPublishPacket publishPacket, CancellationToken cancellationToken)
```

and then, finally DisconnectedAsync handler is triggered:
```batch
warn: Opc.Ua.PubSub.MQTTDataSource[0]
Mqtt client disconnected: NormalDisconnection
```

### Which component is your bug related to?
- Client

### To Reproduce
Steps to reproduce the behavior:
1. Using this version of MQTTnet '4.2.1.781'.
2. See description of the bug.
4. See error.

### Expected behavior
I expect all the PublishAsync tasks to complete with exeption or to be cancelled and client.DisconnectedAsync to be invoked after MqttClientOptions.Timeout or given CancellationToken.