inspector: no URLs when the debugger is connected

By convention, inspector protocol targets do not advertise connection
URLs when the frontend is already connected as multiple inspector
protocol connections are not supported.

PR-URL: https://github.com/nodejs/node/pull/8919


Reviewed-By: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>