WIP: Scaleout plugin

* Enables clients to enable scale out groups to be used for routing

* Enables servers to enlist in a ScaleOut group

#### Things left to do

- [ ] Add support for the gracefull shutdown call back (depends on the core pull)
- [ ] Cache the SC routing table for X seconds
- [ ] How do we acceptance test this?
- [ ] Should we perform retries when we startup and SC isn't responding? 
- [ ] Move the SC api url config to a common `.ServiceConntrolUrl(...)` config call and by default read `AppSettings["ServiceControl/ApiUrl"]`
- [ ] Document the public api on docs.particular.net
I would instead have a background timer keeping the scale out groups up to date.