428232 - Rework batch mode / buffering in websocket.

Better default size for SimpleTextMessage Utf8StringBuilder,
to avoid multiple expansions and data copy of the underlying array.

jetty-websocket/websocket-common/src/main/java/org/eclipse/jetty/websocket/common/message/SimpleTextMessage.java

@@ -34,7 +34,7 @@ public class SimpleTextMessage implements MessageAppender
     public SimpleTextMessage(EventDriver onEvent)
     {
         this.onEvent = onEvent;
-        this.utf = new Utf8StringBuilder();
+        this.utf = new Utf8StringBuilder(1024);
         size = 0;
         finished = false;
     }