swarm/one_shot: Initialize handler with KeepAlive::Until

[mxinden]

The OneShotHandler keep_alive property is altered on incoming and
outgoing reqeusts. By default it is initialized in KeepAlive::Yes. In
case there are no incoming or outgoing requests happening, this state is
never changed and thus the handler keeps the underlying connection alive
indefinitely.

With this commit the handler is initialized with KeepAlive::Until. As
before the keep_alive timer is updated on incoming requests and set to
KeepAlive::Yes on outgoing requests.

---

Related to paritytech/polkadot#1544.

[romanb]

It may be better to just move this condition from inject_fully_negotiated_outbound

        if self.dial_negotiated == 0 && self.dial_queue.is_empty() {
            self.keep_alive = KeepAlive::Until(Instant::now() + self.config.keep_alive_timeout);
        }

into poll()

       ...
       if !self.dial_queue.is_empty() {
            ...
        } else {
            self.dial_queue.shrink_to_fit();
            if self.keep_alive.is_yes() && self.dial_negotiated == 0 {
                self.keep_alive = KeepAlive::Until(Instant::now() + self.config.keep_alive_timeout);
            }
        }

The reason being that starting timers on creation of connection handlers is mildly problematic, because at least right now, handlers are created before the connection is even established, whereas a connection handler is poll()ed as soon as it is established and it gives the handler a chance to "make its first step" before the timeout starts (if there is indeed nothing that the handler wishes to do, as determined by poll()).

[mxinden]

Thanks @romanb for pointing out the early initialization issue! Could you take another look?