RPC connection isolation mechanism needs an asynchronous option for connection isolation function

[eliransin]

There are use cases where isolation of the connection can not be prepared in advance. With only the current API the application has no choice but to fallback to a default (which is not always desired) or to reject the connection. We should have an asynchronous equivalent that will allow the application to create a fiber the resolves with a scheduling group (future<scheduling_group>) that will make the necessary application side configuration prior to returning the scheduling group. A trivial example is that this API will allow the application first creating the scheduling_group and only the returning it.