TimeoutsHolder isn't threadsafe
Causing NPE:

```
java.lang.NullPointerException
    at com.ning.http.client.providers.netty.timeout.TimeoutsHolder.cancel(TimeoutsHolder.java:27)
    at com.ning.http.client.providers.netty.timeout.IdleConnectionTimeoutTimerTask.run(IdleConnectionTimeoutTimerTask.java:66)
```
