Fixed minor at Microservices.CompositeServiceDiscovery.start0

Author: artem-v

services/src/main/java/io/scalecube/services/Microservices.java

@@ -638,6 +638,7 @@ private Mono<? extends Void> start0(String id, ServiceDiscovery discovery) {
       disposables.add(
           discovery
               .listen()
+              .subscribeOn(scheduler)
               .publishOn(scheduler)
               .doOnNext(event -> onDiscoveryEvent(microservices, event))
               .doOnNext(sink::tryEmitNext)