Make scale-out (relay) server a plugin
We want to provide more option in relay server and make them "scale out servers". The main ideas are to allow redoundancy on a pool of relay servers and remove latencies between node and server (even with a bidirectionnal protocol between node and policy servers).
The first step (handled in that ticket) is to split relay code appart from rudder main web application to allows the code to be extended (hopefully) without need to upgrade Rudder version. A plugin will be created in parallel to that ticket.