Openflow plugin statistic scheduler dies under unexpected exception

MD-SAL or any internal module could potentially throw an IllegalStateException exceptions that Openflow plugin can not manage and consequently the thread that is in charge of updating the statistics stops.

We have detected cases which this IllegalStateException exceptions have been produced and flow statistics are not updated anymore while the connection to the Openflow switch is stable and working properly.

In these scenarios, the controller keeps the ownership of the switch with a stable connection but the statistics are not updated anymore.

The source of the problem is related to the following piece of code in StatisticsManagerImpl.java method pollStatistics when defining the onFailure in the callback.

```
if (throwable instanceof IllegalStateException) {
stopScheduling(deviceInfo);
}
```