Ensure proper cleanup of subscribers

In the case of server shutdown, this change ensures that we abort
the tasks responsible for handling client connections before
fully shutting down. This allows the named pipe connections to be
dropped properly. (Fixes #13)

In the case of a client disconnection at any point in the message loop
for the subscriber, the server will ensure that the disconnect is
handled instead of exiting the subscriber handler immediately. This
fixes a rare issue wherein the server does not exit when the last client
disconnects because the disconnection happened while the server is
sending a message to the subscriber.