Iterate over iterable things

...not over RelatedManagers.
Merge pull request #283 from drkitty/vrf-hotfix

Vrf.get_related_networks() hotfix
Revert "Merge pull request #278 from church29/RangeUsage"

We will be implemented range usage in a more efficient way.
At the moment, ranges in global cause over 3000 queries.
This reverts commit ec433bbdfa5b9ea439913ba907756200662e6768, reversing
changes made to dd293ec46d71c983be912830039f424583484ddc.