Remove redundant connection_t::status.active field

The only places where `connection_t::status.active` is modified is in `ack_h()` and `terminate_connection()`. In both cases, `connection_t::edge` is added and removed at the same time, and that's the only places `connection_t::edge` is set. Therefore, the following is true at all times:

    !c->status.active == !c->edge

This commit removes the redundant state information by getting rid of `connection_t::status.active`, and using `connection_t::edge` instead.