The original ServergRPC.Listen() method returns a tls.Listener
when tls protocol is specified. Unfortunate, tls.Listener
does not implement the caddy.Listener interface that is
needed for graceful restart to work.

The change is to have the ServergRPC.Listen() method returns the
inner net.Listener instead. The creation of the tls.Listener
is deferred to the ServergRPC.Serve() method.