accesslib: get_user_access_bycontext() fetches all relevant rdefs

accesslib: get_user_access_bycontext() fetches all relevant rdefs

get_user_access_bycontext() was narrowing down too much the rdefs it
was fetching. With this patch, it now correctly retrieves the rdefs for
new roles assigned in lower contexts, and also correctly retrieves
rdefs present in the course context (fixing MDL-11220).

This also means that we now do the job in 2 DB queries (instead of 3),
and we move a bit more data, but those rows are actually needed ;-)