Major Description
The @PreDestoy method called look like this :
```
for ( String mbean : mbeans) {
unregister(mbean);
mbeans.remove(mbean);
}
```
Causing mbeans being ignored and not unregistered.
Solution is to simply not modify collection content while working on it and simply clear it outside the for loop.
Acceptance criteria :
- Being able to launch a James server, stop it and restart it.