Automated testing of TimeSteward callers about time-steward

[elidupree]

One of the important techniques for testing determinism is to run 2 instances of the same simulation on different devices, and continuously verify that they remain synchronized – even with network delay affecting the fiat events issued by each device. This requires a full TimeSteward implementation, and also some additional features we haven't settled on yet.

[elidupree]

simply_synchronized is now working and successfully caught me using HashSet inappropriately. There is much more work to be done in this area – including making the panic messages more detailed and understandable – but I consider this sufficient for MVP.

[elidupree]

With the API changes, the old testing is no longer applicable, and there are many new tests that need to be done.

My idea is to make an auditing TimeSteward – a separate TimeSteward implementor that does costly operations to detect as many possible incorrect behaviors as possible. I'm thinking it would also be able to take a replay – a record of fiat events plus a record of what order all events were run – and rerun the simulation in that order, even if the original simulation was run by a real-time TimeSteward instead of the auditing one.