Increase number of cleanup threads

Author: fangerer

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/runtime/AsyncHandler.java

@@ -132,7 +132,7 @@ public final void execute(PythonContext context) {
         }
     }
 
-    private final ScheduledExecutorService executorService = Executors.newScheduledThreadPool(4, new ThreadFactory() {
+    private final ScheduledExecutorService executorService = Executors.newScheduledThreadPool(6, new ThreadFactory() {
         @Override
         public Thread newThread(Runnable r) {
             Thread t = Executors.defaultThreadFactory().newThread(r);