Switching to lazyload for getting the next job query

Testing showed that lazyload is apparently fastest in our specific
case since we only fetch a single job here and there's only one
child table, hence only two simple queries vs. subqueryload which
issues as second more complex query or joinedload which issues
a single more complex join.

Removed an unnecessary status == None check since status will
never be None. (Will add this db restraint as well in the migration
commit coming soon.) Test change to reflect this as well.