Make MAX_STRING_NODE_LENGTH configurable (#9995)

Make `ServerSerializationStreamWriter.MAX_STRING_NODE_LENGTH`
configurable, to better work around #9578 and allow real JSON in all
cases.
This PR is a follow-up to #9961 (making the rpc serialization chunk size
configurable).

Workaround #9578 
Followup #9961

user/src/com/google/gwt/user/server/rpc/impl/ServerSerializationStreamWriter.java

@@ -345,7 +345,8 @@ abstract void write(ServerSerializationStreamWriter stream, Object instance)
    * This exists to work around a Rhino parser bug in the hosted mode client
    * that limits string node lengths to 64KB.
    */
-  private static final int MAX_STRING_NODE_LENGTH = 0xFFFF;
+  private static final int MAX_STRING_NODE_LENGTH =
+      Integer.getInteger("gwt.rpc.maxStringNodeLength", 0xFFFF);
 
   static {
     /*