Server and server transport needs to provide closure notification

[stephentoub]

This could be a CompletedTask or similar.

Especially for the stdio transport, the server process logic needs to be able to exit when the client disconnects. It can't rely on the client always doing the "right thing". and killing the server process.

The getting started server code should be able to just await the completion of the stdio server.

cc: @halter73

[halter73]

Fixed by #122/#142