GraphChangedListener can mutate graph without concurrent modification exception.

This change means that a GraphChangedListener can modify the graph without ConcurrentModificationException in the event queue.

Useful if you want to pre-calculate some derived data.

For example:
```java
@Override
public void vertexPropertyChanged(Vertex vertex, String key, Object oldValue, Object setValue) {
  //Care! Avoid an infinite change events being triggered.
  if (!"timestamp".equals(key)) {
    vertex.setProperty("timestamp", System.currentTimeInMillis());
  }
}
```

Also does the Tinkerpop project have a standard for the line lengths in code and comments so I can set up my code formatter?