Remove false positives from channel_is_client()

Remove false positives from channel_is_client()

Fix a set of false positives where relays would consider connections
to other relays as being client-only connections (and thus e.g.
deserving different link padding schemes) if those relays fell out
of the consensus briefly.

Now we look only at the initial handshake and whether the connection
authenticated as a relay.

Fixes bug 24898; bugfix on 0.3.1.1-alpha.