refactor: convert global modulestore to request-cached

[ormsbee]

A more modest version of #37842

Having a global modulestore reference is a holdover from the early days,
when the XML modulestore would be extremely expensive to initialize.
That is no longer the case, and we should eliminate the global here to
reduce the chances of multi-threading bugs that might mutate the global.

Unfortunately, a lot of code currently assumes that calls to the
modulestore() function are essentially free, instead of the 1-2 ms it
really is. There are cases where this may be called thousands of times
deep in loops somewhere while doing course content traversal, so it's
risky to eliminate the caching behavior altogether. The thought here is
that we'll tie to the RequestCache, which should at least not be shared
across users/threads.

[ormsbee]

@robrap and @timmc-edx flagged that 2U has seen operational issues related to having too many MongoDB connections:

@robrap: In the last year we had a production incident where a runaway scaling event brought down production because of too many connections to Mongo. If I remember correctly, it was around 80-100 LMS webapp instances. I don't know how useful this is, but still worth being aware of.
tmccormack

@timmc-edx: And notably, I think this was only with maybe 3-4x as many webapp instances as we normally have.