Annotations: speed up neuron name service back-end

For some reason Django's ORM creates very strange queries if things get a bit
more complex. When requesting annotations for the neurons modeled by a set of
skeletons, a lot of LEFT OUTER JOINs where used while INNER JOINS would have
been perfectly fine. Needless to say that this is a performance hit on large
tables like class_instance_class_instance. Therefore, this commit replaces the
Django ORM query with regular SQL. The query performance has improved quite a
bit and now even longer requests return again.