Changing the signature of the onOverflow function parameter given to DropNewAndSignal, DropOldAndSignal and ClearBufferAndSignal.

We have had a signature like this:

case class DropNewAndSignal[A](bufferSize: Int, onOverflow: Long => A)
  extends OverflowStrategy[A]

The onOverflow is a function that receives the number of elements that were dropped while the consumer gets busy. So the consumer can then be notified that it had dropped events and behave accordingly. However this isn't always helpful. There are cases where we don't want to send any such message to the consumer, but we just want to log this information somewhere. We can do this currently, but it is way too hard and wasteful (e.g. boxing stuff in Either and filtering), so we are changing that signature to this:

case class DropNewAndSignal[A](bufferSize: Int, onOverflow: Long => Option[A])
  extends OverflowStrategy[A]

Thus, you can return Some(a) in case you want to inform the downstream subscribers of the overflow, or you can return a None, in which case no message gets sent. So you can just return None and log stuff.