iproto: check connection state instead of fd

iproto_connection->input.fd != -1 iff state != IPROTO_CONNECTION_ALIVE.
Let's check the state instead of input.fd or output.fd, because it's
easier for understanding.

While we are at it, drop the stale comment to iproto_connection_is_idle:
the function doesn't use evio_has_fd at all.