Fix early subscription completion stage

[gytis]

AMQP and Kafka modules extensively use the following structure: Mono.fromCompletionStage(delegate.methodReturningCompletionStage()). However, this does not postpone execution until the there is an actual subscription to mono.

For example with a still in process Kafka client the following would end up in a successful assignment:

interface KafkaConsumer {
  //...
  Mono<Void> assign(Partition partition);
  //...
}

class KafkaConsumerUser {
  //...
  public void useConsumer(Partition partition) {
    kafkaConsumer.assign(partition);
  }
}

We should replace those constructs with the following: Mono.fromCompletionStage(() -> delegate.methodReturningCompletionStage())