refactor: remove base_time from mono_clock

[Green-Sky]

The unix time offset is not used anywhere and was causing issues on windows.

Please everyone check, I might have overlooked a "unix time" usage.

Also, I not that the function current_time_monotonic() should probably be removed and the only user seems to be net_crypto. This would remove the extra lock from the windows code path and clean up the code alot.

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This change is 

[Green-Sky]

@iphydf the fuzzer reproducibility never return 0 thingy, how do we properly solve it?

[zoff99]

if you want to remove current_time_monotonic() what would be the replacement for it?
i use it a lot for toxav.
removing it without a replacement does not sound optimal.

[Green-Sky]

@zoff99 call update on the clock more often. It is supposed to be a caching clock. Side stepping it is kind of wasteful. Especially on windows, where we have to carry around an extra mutex, just for the "sidestepping".