Removed loop calling the ExecutionStrategy, since it was spinning.

In case of ProduceExecuteRun, the producer is called in a loop, and
it never returns null (unless stopped), so ExecutionStrategy.execute()
never returns.

In case of ExecuteProduceRun, ExecutionStrategy.execute() does return,
but only after it has arranged another thread to continue producing.
Therefore calling ExecutionStrategy.execute() in a loop was spinning
since the ExecutionStrategy had already arranged to produce with
another thread and therefore calling execute() again was a
no-operation hence causing the spinning.

jetty-io/src/main/java/org/eclipse/jetty/io/ManagedSelector.java

@@ -170,11 +170,9 @@ public class ManagedSelector extends AbstractLifeCycle implements Runnable, Dump
     @Override
     public void run()
     {
-        while (isRunning() || isStopping())
-            _strategy.execute();
+        _strategy.execute();
     }
 
-
     @Override
     public Runnable produce()
     {