cells: reduce dependency on ListenableFuture

Motivation:
as java8 provides CompletableFuture there are no reasons to depend on guava's
ListenableFuture.

Modification:
Update CellNucleus#start to return CompletableFuture

Result:
no visible changes.

Acked-by: Olufemi Adeyemi
Target: master
Require-book: no
Require-notes: no

modules/cells/src/main/java/dmg/cells/nucleus/CellAdapter.java

@@ -43,7 +43,6 @@
 import org.dcache.util.Version;
 import org.dcache.util.cli.CommandExecutor;
 
-import static org.dcache.util.CompletableFutures.fromListenableFuture;
 import static org.dcache.util.MathUtils.addWithInfinity;
 import static org.dcache.util.MathUtils.subWithInfinity;
 
@@ -206,7 +205,7 @@ public CellAdapter(String cellName,
      */
     public CompletableFuture<Void> start()
     {
-        return fromListenableFuture(_nucleus.start());
+        return _nucleus.start();
     }
 
     public void addCommandListener(Object commandListener)

modules/cells/src/main/java/dmg/cells/nucleus/CellNucleus.java

@@ -58,6 +58,7 @@
 import static com.google.common.base.Preconditions.checkState;
 import static com.google.common.collect.Iterables.consumingIterable;
 import static com.google.common.util.concurrent.MoreExecutors.directExecutor;
+import static org.dcache.util.CompletableFutures.fromListenableFuture;
 import static org.dcache.util.MathUtils.addWithInfinity;
 import static org.dcache.util.MathUtils.subWithInfinity;
 
@@ -906,7 +907,7 @@ private void setState(State newState)
      *
      * Must only be called once.
      */
-    public ListenableFuture<Void> start()
+    public CompletableFuture<Void> start()
     {
         _lifeCycleMonitor.enter();
         try {
@@ -916,7 +917,7 @@ public ListenableFuture<Void> start()
         } finally {
             _lifeCycleMonitor.leave();
         }
-        return Futures.nonCancellationPropagating(_startup);
+        return fromListenableFuture(Futures.nonCancellationPropagating(_startup));
     }
 
     private Void doStart() throws Exception