Issue #6603 - HTTP/2 max local stream count exceeded

Made newEntry() private to avoid creation of more than 2 Entry subclasses.

Signed-off-by: Simone Bordet <simone.bordet@gmail.com>

jetty-util/src/main/java/org/eclipse/jetty/util/Pool.java

@@ -353,8 +353,10 @@ public Entry reserve()
         }
     }
 
-    protected Entry newEntry()
+    private Entry newEntry()
     {
+        // Do not allow more than 2 implementations of Entry, otherwise call sites in Pool
+        // referencing Entry methods will become mega-morphic and kill the performance.
         if (maxMultiplex >= 0 || maxUsage >= 0)
             return new MultiEntry();
         return new MonoEntry();