Add a CancellableQueue to make close methods safer

There are multiple locations in which, on a call to close within some portability service, we wish to close all of our outstanding clients. However, the call to close can be interleaved with calls to the method which creates a new client. Without having an explicit thread-safe signal from the collection of pending clients, there's no way to ensure that all of the clients for a service are closed when that service is closed. 

As a result, some clients are only terminated by the forceful termination of the server, killing the connection.