windows: deal with the fact that GetTickCount might lag

We use GetQueuedCompletionStatus(Ex) to sleep the thread until the next
timer expires (provided that no other events happen before that).
However after waking up from a sleep the GetTickCount() return value may
not immediately reflect that some time has passed. This happens because
gqcs can sometimes sleep for periods shorter than the GetTickCount clock
resulution. This patch changes time tracking so the amount of time
waited by gqcs is taken into account.

This has the following advantages:

* Excessive loop iterations are avoided.
* Small timeouts are fired more precisely.
* The `loop-stop` test that used to be flaky on Windows now passes
  consistently.

@bnoordhuis or @sblom -> review?