Adjust caching for `get_term_by()` calls:

* Remove md5 hashes for term name cache keys
* Remove the namespace for the keys for `names` and `slugs` and add them to the group names
* Remove `wp_get_last_changed()`, which @nacin hated
 
Props tollmanz.
Fixes #21760.


git-svn-id: https://develop.svn.wordpress.org/trunk@30073 602fd350-edb4-49c9-b593-d223f7449a82