tone down error logging in IO layer, see #3386

* Made defaultDecider available in SupervisorStrategy,
  turned out that I didn't need it but I think it could be
  good anyway, e.g.
  override def supervisorStrategy = OneForOneStrategy(
    enableLogging = false)(SupervisorStrategy.defaultDecider)
* Verified the following scenarios:
  - client connection failure
  - server bind failure
  - kill client (peer closed)
  - kill server (peer closed)