Gardening: Simplify Node.addService

Use Map.computeIfAbsent()

Change-Id: I44b64b682645fadadcc89642c63e939c5691c091
Reviewed-on: https://review.couchbase.org/c/couchbase-jvm-clients/+/208839
Reviewed-by: Michael Reiche <michael.reiche@couchbase.com>
Tested-by: Build Bot <build@couchbase.com>

core-io/src/main/java/com/couchbase/client/core/node/Node.java

@@ -233,11 +233,8 @@ public synchronized Mono<Void> addService(final ServiceType type, final int port
       }
 
       String name = type.scope() == ServiceScope.CLUSTER ? GLOBAL_SCOPE : bucket.orElse(BUCKET_GLOBAL_SCOPE);
-      Map<ServiceType, Service> localMap = services.get(name);
-      if (localMap == null) {
-        localMap = new ConcurrentHashMap<>();
-        services.put(name, localMap);
-      }
+      Map<ServiceType, Service> localMap = services.computeIfAbsent(name, key -> new ConcurrentHashMap<>());
+
       if (!localMap.containsKey(type)) {
         long start = System.nanoTime();
         Service service = createService(type, port, bucket);