Poor performance when XBlock class is not found #811
Description
Checking the entrypoints for the XBlock class we want for a given tag name is relatively expensive. We mitigate this by caching all the XBlock classes when we load them. But we don't cache when we fail to find an XBlock class. So if a course has many instances of a type of XBlock that is not installed on the given instance, we're constantly scanning entrypoints. This really slows down things like the Studio course outline page.
The fix is to cache the fact that there is no matching XBlock class for this tag.