accesslib: get_user_access_bycontext() fetches overrides in subcontexts, intro aggr_r...

accesslib: get_user_access_bycontext() fetches overrides in subcontexts, intro aggr_roles_fromsess()

The main thing here is that overrides in subcontexts are
now read correctly (and still cheaply) into the access array.

Also

 - introducing aggr_roles_fromsess() with gives you a list of
   the relevant roles that affect the user in this context

 - we clear out $USER->access on login/logout

 - get_user_access_bycontext() gets a few optimisations too...