Any incoming connection that receives a replSetHeartbeat command gets marked with the tag transport::Session::kKeepOpen, and any incoming connection that receives an ismaster command with the field "hangUpOnStepDown" set to true gets tagged with executor::NetworkInterface::kMessagingPortKeepOpen. Those tags both have the same value (1), so they both have the same affect of preventing that connection from being closed on stepdown from primary. We should dedupe these two and be consistent in how we refer to that tag/behavior.