Set max depth for JSON serializer to mitigate known DOS vulnerability

The other option is to update Newtonsoft.Json, which now also sets the
maximum depth by default, but this mitigates without having to update.

Author: andyleejordan

src/JsonRpc/Serialization/SerializerBase.cs

@@ -19,7 +19,7 @@ protected virtual JsonSerializer CreateSerializer()
 
         protected virtual JsonSerializerSettings CreateSerializerSettings()
         {
-            var settings = JsonConvert.DefaultSettings != null ? JsonConvert.DefaultSettings() : new JsonSerializerSettings();
+            var settings = JsonConvert.DefaultSettings != null ? JsonConvert.DefaultSettings() : new JsonSerializerSettings { MaxDepth = 128 };
             AddOrReplaceConverters(settings.Converters);
             return _settings = settings;
         }