fix #1335 auto update wpcom blog list

fix #1335 auto update wpcom blog list (maximum rate every 6 hours).
Could you please check this function uses the correct keys when doing comparisons with Maps?
I see that `newBlogURLs` is initialized by using the keys `xmlrpc` and `blogid`, but checked later with the keys `url` and `blogId`. <--- It's probably correct, but better to double check this.