Add mechanism to mark simulations as charged to avoid charging more than once

Related tasks

• Avoid charging users more than once if cloudsim fails to shut a simulation down

Context

Whenever cloudsim fails to tear a simulation down, sysadmins usually request a manual termination of that simulation to go over the stop simulation action once more. The problem is that users are being charged more than once when that request is processed, dealing to duplicated charges.

Change

This MR adds a ChargedAt field to keep track when a simulation has been charged. This only applies to simulations that is being processed in the stop simulation action, and not in any rollback handler.

Other information

Choosing ChargedAt over PriceCharged because ChargedAt required less work to be implemented. PriceCharged require us to update a simulation in the SubtractCredits, making the Billing service depend on the Simulations service.

Additional documentation