Return CompletableFutures for public-facing operations

[jchambers]

As a follow-up to #746, this pull request replaces all public-facing uses of io.netty.util.concurrent.Future with java.util.concurrent.CompletableFuture. CompletableFuture is part of the stock Java SDK and offers all of the affordances—most importantly being able to add listeners—that Netty's Future offers.

[jchambers]

I should also point out that this pull request highlights how much non-IO stuff we're inadvertently doing on IO threads. I'll open a follow-up pull request to add an "insulation" layer.

[jchambers]

Also, I'm trying to figure out how to describe the action of calling CompletableFuture#thenApply(...). "Add a listener" seems imprecise since CompletionStage is its own concept in the Java world, but "add a dependent completion stage" also feels a little obtuse for folks who aren't already familiar with CompletableFuture/CompletionStage. I'm rolling with "add a follow-up task" for now, but would love suggestions for alternative phrasing!