We should never be using CompletableFuture.completionStage

When we upgraded to JDK 11 we now have access to a new method on CompletableFuture named completedStage. This is very similar to completedFuture, but prevents Future based methods and toCompletionStage causes a new allocation.

This can be problematic as many places use toCompletableFuture for already completed stages to prevent lambda and other allocations.

We need to add a code style check to prevent this as well, as many people may not notice using the wrong method.