Kotlin support

We should have extension functions for skipping java's function interfaces and going for kotlin's dsl functions... Something like:
```kotlin
val scheduler: ServerImplementation = null
scheduler.async().runDelayed(4L) { task ->
// do stuff
// maybe return stuff
}
```

Start by inspecting the existing scheduler API and its Java function-interface overloads; the issue names no files or tests. Clarify the scope of Kotlin extension functions and the expected behavior of the shown async().runDelayed(4L) DSL before treating the feature as complete.

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