Revert "reduce minimum AsyncHandler pool size to 1"

This reverts commit 4ef125ea5a5c6f10ff483c700818052b22822fd8.

Author: fangerer

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

@@ -143,7 +143,7 @@ public final void execute(PythonContext context) {
         }
     }
 
-    private final ScheduledExecutorService executorService = Executors.newScheduledThreadPool(1, runnable -> {
+    private final ScheduledExecutorService executorService = Executors.newScheduledThreadPool(6, runnable -> {
         Thread t = Executors.defaultThreadFactory().newThread(runnable);
         t.setDaemon(true);
         return t;