Stop executors to speed shut down of operator

Author: rjeberhard

operator/src/main/java/oracle/kubernetes/operator/BaseMain.java

@@ -237,6 +237,7 @@ void waitForDeath() {
 
     acquireShutdownSignal();
 
+    executor.shutdown();
     stopAllWatchers();
   }
 

operator/src/test/java/oracle/kubernetes/operator/work/FiberTestSupport.java

@@ -242,6 +242,11 @@ int getNumItemsRun() {
       return numItemsRun;
     }
 
+    @Override
+    public void shutdown() {
+      // no-op
+    }
+
     @Override
     @Nonnull
     public ScheduledFuture<?> schedule(