feat: completion function to MessageConsumer returns a future

[wem]

trafficstars

feat: completion function to MessageConsumer returns a future

Alternative to completionHandler.

Motivation:

https://github.com/vert-x3/vertx-lang-kotlin/issues/190

[vietj]

can you rebase on latest master ?

[vietj]

I think the main issue here is that a consumer can be registered or unregistered multiple times and it is not clear what a registration future would mean, we need to clarify this before taking any action

[tsegismont]

Superseded by #4645