Unknown memory leak

### Describe the bug
Memory usage runaway.

### Which component is your bug related to?
Server

### To Reproduce
Steps to reproduce the behavior:
1. Using version v4.3.0.858
2. Set up a simple server application with the following events
ValidatingConnectionAsync - Checks clients against database
InterceptingPublishAsync - Just to increase a counter to keep stats on number of msg handled
LoadingRetainedMessageAsync - Uses code from samples
RetainedMessageChangedAsync - Uses code from samples
3. The server has the following options:
```
var options = new MqttServerOptionsBuilder().
WithPersistentSessions(true).
WithConnectionBacklog(2000).
WithDefaultEndpointPort(1883).
WithDefaultEndpoint().
WithKeepAlive().
WithMaxPendingMessagesPerClient(20000);
```
4. See error.

### Expected behavior
The memory should vary with the number of connected clients and number of persistent messages in memory.

### Screenshots

### Additional context / logging
The server also has a timer, that once a sec updates the GUI with info about: connected clients, numb of processed msg and msg/sec.
Before I also displayed all connected clients in a ListView (WPF). Has worked without issue, but that was before everything was made asynchronous.

I made a client-app that spams the server with both connections and messages that should be persistent to try to
find the leak. I have not used the memory profiler before, but if I understand it correctly it has something to do with
getting the connected client count.

Let me know if I'm missing something.