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.
So actually, this would be more correct, and nicer if you ask the reactor for when it will wake up, instead of the scheduler for when the next timer will fire.

```ruby
unless @io_loop.next_tick_before?(@clock.now + timeout)
  @unblocker.unblock
end
```