Don't use Scala Futures in Java APIs

[raboof]

Sketching out #1417 - incomplete and notably not bothering with binary compatibility yet, just to illustrate the idea.

[mdedetrich]

Not sure if this is related but there are methods like actorSystem.terminate which return a Future which should instead for Java be returning a completion stage (although I am not sure of what the nice way of doing this is since there isn't an actor system dsl).

[He-Pin]

@raboof Would you like to continue this? AsyncRecoveryPlugin and etcs should use Java's CompletionStage.

[pjfanning]

#2092 took most of what was in this PR.