Suggest to Edge that it should drop connection

Sending the 'Connection: close' header SHOULD mean that Varnish
will drop its persistent connection to the backend after receipt.

This has been tested successfully, so StopServer should now be
useful in our failover tests.
Make Stop() a function on our Mux

... rather than a main function that takes a mux as a parameter
Explicitly request the Close handler

We do not want to rely on a subsequent request calling the Connection: close
and hence causing the Keep-Alive connection to drop.

Hence, ensure we make this request ourselves, directly after switching in the handler.