gossipd: drain fd instead of passing around gossip index.

(This was sitting in my gossip-enchancement patch queue, but it simplifies
this set too, so I moved it here).

In 94711969f we added an explicit gossip_index so when gossipd gets
peers back from other daemons, it knows what gossip it has sent (since
gossipd can send gossip after the other daemon is already complete).

This solution is insufficient for the more general case where gossipd
wants to send other messages reliably, so replace it with the other
solution: have gossipd drain the "gossip fd" which the daemon returns.

This turns out to be quite simple, and is probably how I should have
done it originally :(

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

lightningd/onchain_control.c

@@ -357,7 +357,6 @@ static bool tell_if_missing(const struct channel *channel,
 static void onchain_error(struct channel *channel,
 			  int peer_fd UNUSED, int gossip_fd UNUSED,
 			  const struct crypto_state *cs UNUSED,
-			  u64 gossip_index UNUSED,
 			  const struct channel_id *channel_id UNUSED,
 			  const char *desc,
 			  const u8 *err_for_them UNUSED)