Optimize server-side LinkedHashMap_CustomFieldSerializer.

1) Don't keep trying reflection if it's not working.
2) Cache the reflective Field needed for accessOrder.
3) Reuse KEY objects instead of making new ones each time.

http://gwt-code-reviews.appspot.com/829802/show
Review by: rice


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@8777 8db76d5a-ed1c-0410-87a9-c151d255dfc7