Fix issue #1187 and #1196

[zsxwing]

This PR fixed issue #1187.

Although using (action: => Unit) will require the following odd codes when somebody wants to use Function0 directly:

  @Test def schedulerExample5(): Unit = {
    val worker = IOScheduler().createWorker
    var no = 1
    val subscription = worker.schedule(new Function0[Unit]{
      def apply(): Unit = {
        println(s"Hello(${no}) from Scheduler")
        no += 1
        worker.schedule(apply(), 100 millis)
      }
    }.apply(), 1 seconds)
    TimeUnit.SECONDS.sleep(2)
    subscription.unsubscribe()
  }

I think people rarely use Function0 directly in Scala.

So I chose (action: => Unit) finally. schedulerExample4 is an more realistic example for recursive schedule.

This PR is a breaking change.

[zsxwing]

/cc @headinthebox @samuelgruetter

[headinthebox]

Perhaps we should consider putting the "work" as the last (curried) argument.

worker.schedulePeriodically(
 {
   println(s"Hello(${no}) from Scheduler")
   no += 1
}, initialDelay = 1 seconds, period = 100 millis)

Would look very nice like this

worker.schedulePeriodically(initialDelay = 1 seconds, period = 100 millis){
   println(s"Hello(${no}) from Scheduler")
   no += 1
}

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[zsxwing]

Yep. This Looks better.

[headinthebox]

I always try to use that pattern with =>Unit.

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[zsxwing]

Also fixed #1196 in this PR

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