Windows: update loop time lazily

We call uv_update_time() whenever we think the loop time should be updated.
But we might not need to read the time after every update, and the update
operation has non-negligible cost.
So I am making uv_update_time() invalidate the loop time. The time will be
actually updated only on the first call to uv_now() after it's been
invalidated.
It should be initialized, but probably not for the same reasons as pointed out in this comment. I'll just remove the comment, and leave the initialization, which is pretty uncontroversial.