NetworkClock: add manually advanced clock for deterministic tests

[rpaulo]

• Replace configurable-initial-value clock with a manual time backing
  store that tests can freeze and advance by hand
• Route now and nowAbsolute through manual time when set, falling
  back to the system clock otherwise
• Track continuous and absolute time separately, preserving nanosecond
  resolution beyond the system clock's microsecond truncation
• Add tests covering freezing, advancing, and restoring the clock

[tfpauly]

Very cool! Mainly just questioning if we can use a better availability check.

[tfpauly]

Seems a bit odd to tie this to debug logging — I assume this is just intended to compile stuff out on prod / release builds?

[rpaulo]

Yeah pretty much to avoid the memory cost of adding a class. The CPU cost of the extra check in now() isn't that bad.

[tfpauly]

Can we tie it being a debug build or some other trait? I don't think it necessarily makes sense to have this be the logging trait.

[rpaulo]

We can't tie it to a debug build because we run tests in release mode AFAIK.
I didn't want a trait just for this, that's why I used the debug logging trait which is always enabled during the tests

[rnro]

I don't think this should be tied to a Trait at all, it should just use a new Define. Traits are used to allow consumers of a package to modify functionality (by enabling a Define in that package). This only impacts our tests, which consumers of a package don't need to care about, so it should just be a local Define.

[rpaulo]

!DisableDebugLogging is ON by default, that's why I used it. We can make it a define, but it should be on by default so that we don't have to manually turn something on just to be able to run the tests locally.

[rnro]

Yeah I think that's fine personally. Toggling it from the default position becomes an optimization.