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.
Merge pull request #333 from rackerlabs/use_lazyload

Switching to lazyload for getting the next job query