StatCollector is not thread safe and some stats are being mixed up

The instance variable polledSessions is not thread-safe (i.e. uses ArrayList) and its access is not always protected (e.g. StatCollector$Worker#run()). Under heavy load this is producing an error in the Worker.

The bytesReadThroughput statistic is incorrectly being set. It is adding up the throughput of written messages.

Instance variables: msgWrittenThroughput, msgReadThroughput, bytesWrittenThroughputand bytesReadThroughput could be corrupted and return an unexpected value since they are not thread safe and 2 different threads may be changing them.