FluxBufferTimeOrSize buffer can be concurrently modified
This for example causes the `WorkQueueProcessorTest#highRate` to fail:

```
@Override
public void onNext(List<String> strings) {
  int size = strings.size();
  counter += size;
  if (strings.contains(s)) {
    synchronized (s) {
      //logger.debug("Synchronizer!");
      s.notifyAll();
    }
  } //added to showcase the bug:
  else if (size > 1) {
    System.out.println(strings.contains(s));
  }
}
```
The bug manifests itself in the else branch above, where the sysout would print `true` (contradicting the if/else condition).