-complete: All existing background subscriptions need to pass in "phase"
https://github.com/openpeer/op-stack-cpp/issues/56

- started: Introduce chained backgrounding subscriptions
https://github.com/openpeer/op-services-cpp/issues/29

- added: Each backgrounding subscriber should use a setting for attached phase
https://github.com/openpeer/op/issues/70

- added: Proxy subscription mechanism needs to be extended to support sending subscription object in delegate callback
https://github.com/openpeer/op/issues/71