Add consensus param to enable network re-entry at Exit relays

Following the #2667 (closed) work, we want a consensus parameters that control this so in case we made a mistake or things start to go bad, we can disable it through the consensus.

changed milestone to %Tor: 0.4.5.x-stable

added Relay label

Spec: torspec!27 (merged)

mentioned in merge request !287 (merged)

closed with commit 2c3c30e5

mentioned in commit ce3af5dd