Stop Heartbeat timers before disposing them, to prevent any further callbacks and allow background threads to exit

Author: david-poirier

projects/client/RabbitMQ.Client/src/client/impl/Connection.cs

@@ -987,10 +987,12 @@ protected void MaybeStopHeartbeatTimers()
         {
             if(_heartbeatReadTimer != null)
             {
+                _heartbeatReadTimer.Change(Timeout.Infinite, Timeout.Infinite);
                 _heartbeatReadTimer.Dispose();
             }
             if(_heartbeatWriteTimer != null)
             {
+                _heartbeatWriteTimer.Change(Timeout.Infinite, Timeout.Infinite);
                 _heartbeatWriteTimer.Dispose();
             }
         }