refactoring Guava ListenableFuture to Java 8 CompletableFuture

Hi, I'm doing research on new concurrent constructs in Java 8. I found `CompletableFuture` in Java 8 has the same functionality and semantics as Guava `ListenableFuture`. But `CompletableFuture` is much nicer because it comes together with Lambda expression in Java 8, and it is monadic, which makes the code more readable and cleaner. Also, it provides more ways to compose different async tasks. Therefore, using `CompletableFuture` instead of `ListenableFuture` is better for future extension and maintenance of the code.

I tried to refactoring `ListenableFuture` to Java 8 `CompletableFuture` in `redis-protocol`, so you can see the diff. I'm just wondering your opinion on this kind of refactoring (or migrating the code from Java 7 to Java 8). Do you think the refactoring is useful? Do you have any plan to use Java 8 and merge this pr?