Issue 12090 - Make std.concurrency compatible with fibers as threads

https://d.puremagic.com/issues/show_bug.cgi?id=12090
You are right, `Clock.currTime` is similar to `gettimeofday`. However, I don't think that this is the correct API to use here, as it deals with calendar dates/times. Trying to use it for determining intervals will lead to problems when there happens to be a leap second or the system time is suddenly changed (say, by an NTP update with a large delta). I'm pretty sure `clock_gettime(CLOCK_MONOTONIC, …)` is what you want to use here, i.e. `currSystemTick` or `std.datetime.StopWatch`.