Select timer timeout

The default `tick_resolution` timeout is 1 second which means that timers that are scheduled under periods when no IO activity is present will only get the chance to run once every second. This pr adds a bit of intelligence to the value of timeout in the `IO.select`call. Specifically if timers are scheduled we will disregard the `tick_resolution` and only wait for IO until it is time to run the first timer.

When scheduling timers from outside the Io::Reactor the reactor may currently be fast asleep in a `IO.select` call. Therefore we should unblock and run `scheduler.tick` once more to run the recently added timer if the added timer timeout is shorter than the earliest present timer or if there is no present timer.
I think it would be better to ask the IO loop when it will return from `IO.select`.