Add cache invalidation to get_blogs_of_user()

[ryan]

Instead of doing proper cache invalidation, get_blogs_of_user() sets a 5 second timeout on its cache. This results in lots of unnecessary queries. get_blogs_of_user() should set its cache without expiration and do invalidation where needed. Also, it should cache on a list blog ids. There is no need to cache what the get_blog_details() cache is already caching.

[ryan]

(In [15833]) Cache invalidation for get_blogs_of_user(). fixes #14379

[ryan]

(In [16225]) Prime blogs of user cache from old cache to avoid mass DB requests on upgrade. see #14379