AbstractEvent::hasDelegates() is not thread-safe

```
bool hasDelegates() const 
{ 
    return !(_strategy.empty()); 
} 
```

vs. 

```
bool empty() const 
{ 
    typename TMutex::ScopedLock lock(_mutex); 
    return _strategy.empty(); 
 }
```

`hasDelegates()` should simply call `empty()`:

```
bool hasDelegates() const 
{ 
    return !empty(); 
} 
````
