Thanks @zsxwing and @JakeWharton for the input. I agree with you. Unfortunately it will be a (probably long) while before we can use `Duration` but I prefer to stick to the current Java approach and not add our own. 

We definitely do not want to hardcode to nanosecond resolution though like done in this PR: https://github.com/Netflix/RxJava/pull/519

We should include both `long delayTime, TimeUnit unit` parameters wherever they are needed. We have `Func` classes with the number of arguments we need, so let's proceed in that direction.