Avoid ConcurrentModificationException in CloudSim.runClockTick()

[swghosh]

Fixes: #187

ConcurrentModificationException can occur when SimEntity changes it's own state during run() depending upon implementation in between the Simulation clock ticks. This causes to break the simulator run similar to what happens today in CloudSimExample8. Although, this change fixes the said example it now allows users to write their SimEntity implementation in a way that during processEvent / run etc. during the lifecycle of the entity processing it can change it's own state, i.e. change the values of it's linked variables; from a design perspective of the simulator this may/may not be allowed.

In case of CloudSimExample8, the GlobalBroker implementation changes it's state during the course of it's run which today causes the exception to occur so alternative pathway is that we might need to fix the Broker implementation.

[swghosh]

cc: @Astabol @Raove

[Raove]

This fixed the error and the example runs correctly now. Code is basically the same, it just Calls the entity at location i instead of using the entity that can change.