Deprecate ListenableFuture in favor of CompletableFuture

[rstoyanchev]

For application facing APIs, we can provide CompletionStage alternatives. For internal usages, we could retain it or phase it out completely.

[sdeleuze]

+1 for phasing it out, that would improve clarity for users to not have too much alternatives, and with CompletionStage and Mono I think we are covered.