Fix incorrect CompletableFuture integration

[Daenyth]

The cancellation logic is much more complex than the previous implementation handled

This will work correctly with the changes to Async in CE 3.5+

[Daenyth]

I also tried running sbt taglessGen but it looks like current state of the files was manually edited using that as a base - it doesn't compile as-is, there's duplicate methods emitted and a lot of noise in the imports

[semenodm]

you are right, i didn't polish codegen. every time i update sdk versions, i do codegen and then manually fix imports and method overload issues.

thanks for the contribution, this makes sense.

[semenodm]

merging this guy, thank you for the contrubution @Daenyth