Replace Task.Delay, which has only 15ms or so resolution, with an alternative, high-performance timer

[abelbraaksma]

Fixes #15.

It appears that the issue in #15 is caused by how Task.Delay is highly imprecise (with, depending on system, a 15ms resolution). Replacing it with what seems to be a possible high-resolution alternative (see this IoT helper code: dotnet/iot#235), which uses SpinWait and SpinOnce to delay the task, the "performance" issues disappeared as snow in the sun.

The expected timings of these smoke-tests should be roughly the amount of tasks divided by the average random delay configured for each task (this is a test feature only!).

It brings the offending tests from 15s runtime or more down to 600ms, with double the amount of test-tasks.

There are still some questions, mainly why does the spinwait.SpinOnce cause a 15ms delay per task on average?

TLDR: we now have a microsecond task-delay timer 😆!

[abelbraaksma]

There are still some questions, mainly why does the spinwait.SpinOnce cause a 15ms delay per task on average?

About that: it turns out that SpinOnce internally uses the same timer as Task.Delay, and that it uses Thread.Sleep(0) to yield and Thread.Sleep(1) to block every couple of spins.