Fix cache stomping in get_bookmarks()

[josephscott]

The cache in get_bookmarks() can stomp on itself, resulting in cache misses even after adding it to the cache.

I'm including a patch that breaks the cache up based on the value of $category.

[markjaquith]

Stomping because of high concurrency of cache adds?

[ryan]

category is part of key, shouldn't that take care of it?

[josephscott]

After looking at this further the diff has problems and really isn't needed.

My initial thought was that the cache was getting stomped because it wasn't breaking it out into enough different groups. This was true, but not really the issue.

The problem is using wp_cache_add, which is fine if you are storing one "thing", but we have an array of things that change. So we should be using wp_cache_set.

[ryan]

(In [5879]) Set instead of add since the cache value has multiple keys that can change. Props josephscott. fixes #4743