accesslib: get_user_courses_bycap() is less of a piggy now...

accesslib: get_user_courses_bycap() is less of a piggy now...

get_user_courses_bycap() replaces get_courses_bycap_fromsess().

Using a combination of in-DB enrolments and in-session capability
checks, we narrow down the courses we fetch from the DB for checking.

This patch adds a small DB query, and has has a moderate impact on
the timings observable on my laptop (~300ms?), but reduces
*significantly* the bandwidth used, which in cluster environments
with frontends separate from backends has a serious impact.