Make XBlock views/handlers non-atomic requests.

We're seeing slow commits on production for courseware_studentmodule
updates. Based on the slow queries during those times, we think it
might be because multiple worker processes are trying to update the
same rows from within long-running transactions (since courseware
is relatively slow).

The risk with this is that since the whole view execution is no
longer wrapped in a big implicit transaction, it's possible that
XBlock state will update and things that key off of that (e.g.
completion progress information or pre-req milestones) will fail
in a way that will leave the database in an unplanned-for state,
though this is already the case for those actions that trigger
asynchronous tasks like grades recalculation.

The query counts for the index view test were adjusted down
because save points count towards the total and we're no longer
setting them at the top level around the view as a whole.