Suspected ReadOnlyTransaction leak in NeutronvpnManager (NeutronvpnUtils)

As found in see NETVIRT-883 attachments, in a relatively low volume Rally test there are many thousands of transactions that are opened in NeutronvpnUtils.read() from NeutronvpnManager.addToNeutronRouterInterfacesMap & removePortsFromSubnetmapNode & removeFromSubnetNode & removeSubnetFromVpn & getVpnForRouter & addToNeutronRouterInterfacesMap & addSubnetToVpn & removeSubnetFromVpn & handleSubnetsForExternalRouter & addSubnetToVpn etc. etc.

While based on my current understanding these could not be the culprits for CONTROLLER-1756 as far as I know this may be causing other memory leaks in the future, and it would be good if these transactions could all be closed correctly.