Use Lwt_switch instead of Lwt_condition to stop the server loop

[dinosaure]

See #339, /cc @talex5:

Hmm, there is a Conduit_lwt.serve. But it returns a Lwt_condition.t to stop it. That's racy - if you broadcast on stop before the loop calls wait then it will miss the event! And it really shouldn't be calling wait each time around the loop.

The normal Lwt way to do that would be to take Lwt_switch argument and abort the loop when the switch is turned off.
A switch can only be turned off once and so you can't miss the event. Also, the switch argument can be optional in the common case where you never want to stop.