Make SystemTime mockable via object

[gretchenfrage]

• Adds trait SystemTimeClock
• Adds default implementation RealSystemTimeClock
• Adds ServerConfigParameter system_time_clock: Arc
• Replaces all SystemTime::now calls in proto with SystemTimeClock.now calls (there were 2)

This is a backwards-compatible change.

[djc]

@aochagavia given that you've used Quinn in simulated time environments, any feedback on this?

[aochagavia]

@djc I haven't needed something like this at all, since as far as I know quinn fully supports mocking time through tokio. In fact, tokio's time mocking features are incredibly powerful, allowing you to automatically time-warp to the next event if the runtime isn't blocked on any other operation. I'd advice anyone needing clock mocking to look into tokio::time::pause and tokio::time::advance.

Update: I see now that this is about SystemTime, so the paragraph above doesn't totally apply (I was thinking of Instant). It looks like I might eventually need something like what this PR proposes, but I'm not fully sure yet.

[gretchenfrage]

Update: I see now that this is about SystemTime, so the paragraph above doesn't totally apply (I was thinking of Instant). It looks like I might eventually need something like what this PR proposes, but I'm not fully sure yet.

Yeah, I believe proto already doesn't have any occurrences of Instant::now(), and that they're all passed in through method params. Possible I missed one though.

[djc]

Thanks!