This is the pointer to the course structure document that is the current published version.
These are NOT immutable
This happens multiple times per second globally (peaking at ~100-125 times per second per course).
So we would give it a cache life of a second or two, then every couple seconds we can update the cache.
Note that we will get a stampede of these as workers spin up. Though we anticipate this should be fine because memcache can handle that behavior.