AutoBeans: synchronize static Maps used for caching

Contributed by tbroyer
Fixes issue 7667
Review at https://gwt-code-reviews.appspot.com/1829804/

Review by: cromwellian@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@11268 8db76d5a-ed1c-0410-87a9-c151d255dfc7