Potential memory leak in Transport.start_server

I'm not sure if this is a leak or an ambiguity on how to use the async version of Transport.start_sever with event being passed, and how to synchronize/accept the client on the server transport.

See https://gist.github.com/mdellavo/9dd98e6223c6f815ad45 -- this version passes an event to Transport.start_server and does not call Transport.accept(), after the transport disconnects, we see that there is an unaccepted Channel in Transport.server_accepts that is now disconnected. Due to the circular reference held between Channel and Transport , this causes a memory leak.

We've also seen in our production code that calling Transport.accept() will return "channel closed" on the client, though this may be unrelated.

See https://gist.github.com/mdellavo/d349cbc9ffc3b30c1862 -- this version does uses the synchronous version (no accept), calls Transport.accept() and functions normally.

Start with the async Transport.start_server path described in the first gist, then inspect Transport.server_accepts, Transport.accept(), and the Channel/Transport references during disconnect. Reproduce the unaccepted-channel case and compare it with the synchronous example; done means the retention behavior and accept semantics are resolved and covered by a regression check.