Add @NonNullApi and @Nullable annotations

[acogoluegnes]

Per discussion with @simonbasle.

[vanseverk]

Is this something I can take onto me? What's the goal?

[acogoluegnes]

@vanseverk Sure, go ahead. The idea is to add @reactor.util.annotation.NonNullApi to the reactor.rabbitmq package, which means that all parameters and return values are non-null and then use @reactor.util.annotation.Nullable in places they can be null.

Maybe @simonbasle has more recommendations.

[simonbasle]

@acogoluegnes @vanseverk that's pretty much it: non-null by default (eased by putting the annotation on the package-info.java) and mark nullable methods (return types and input parameters)

[acogoluegnes]

Closed in #113.