Kotlin/Java interop: let OnPreInit/... work with Java Consumer and kotlin (T) -> Unit

[Jan Galinski]

When we use (T) -> Unit, java users must write return Unit.INSTANCE to fulfill the contract.
To avoid this, we built in a hard reference to java 8 Consumer {}

It would be nice to have a better solutuin if you are using the lib from kotlin, but for now its ok and kotlin users can still work with it.