Use per-instance Mono_Time instead of a global unix_time

[iphydf]

This avoids some global state.

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This change is 

[nurupo]

I'm so confused, there is no explanation either in PR or the commit message as to why this is needed, what improvement this brings. At first I thought that you moved the Windows'es global state into a per-instance struct, but that's not the case.

Also, what is the point of the base_time?

[nurupo]

Oh, nevermind, diff of mono_time.c explains it all. You just removed the the global state struct, nothing else changed, the base_time was there alredy.

second thoughts

[zugz]

I'm having second thoughts on this. Could you explain why this is a positive change?

Mono_Time is meant to represent actual unix time. This is a global concept, so doesn't it actually make sense to use a global for it? Meanwhile making it per-instance increases code complexity.

Having it as a global also makes time manipulation in tests easier.

The only practical advantage I see of making it per-instance is to allow us to model relative clock differences between instances in tests, which I don't anticipate us actually wanting to do.

[iphydf]

@zugz PTAL. I've added a comment to explain the assumptions we want to make in the event loop.

[zugz]

Thanks. I'm happy now.

[zugz]