closing_control: always prefer lower fee, not closest to ideal.

We had an intermittant test failure, where the fee we negotiated was
further from our ideal than the final commitment transaction.  It worked
fine if the other side sent the mutual close first, but not if we sent
our unilateral close first.

ERROR: test_closing_different_fees (__main__.LightningDTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "tests/test_lightningd.py", line 1319, in test_closing_different_fees
    wait_for(lambda: p.rpc.listpeers(l1.info['id'])['peers'][0]['channels'][0]['status'][1] == 'ONCHAIN:Tracking mutual close transaction')
  File "tests/test_lightningd.py", line 74, in wait_for
    raise ValueError("Error waiting for {}", success)
ValueError: ('Error waiting for {}', <function LightningDTests.test_closing_different_fees.<locals>.<lambda> at 0x7f4b43e31a60>)

Really, if we're prepared to negotiate it, we should be prepared to
accept it ourselves.  Simply take the cheapest tx which is above our
minimum.

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

lightningd/closing_control.c

@@ -21,8 +21,7 @@ static bool better_closing_fee(struct lightningd *ld,
 			       struct channel *channel,
 			       const struct bitcoin_tx *tx)
 {
-	u64 weight, fee, last_fee, ideal_fee, min_fee;
+	u64 weight, fee, last_fee, min_fee;
-	s64 old_diff, new_diff;
 	size_t i;
 
 	/* Calculate actual fee (adds in eliminated outputs) */
@@ -49,23 +48,9 @@ static bool better_closing_fee(struct lightningd *ld,
 		return false;
 	}
 
-	ideal_fee = get_feerate(ld->topology, FEERATE_NORMAL) * weight / 1000;
+	/* Prefer lower fee: in case of a tie, prefer new over old: this
-
+	 * covers the preference for a mutual close over a unilateral one. */
-	/* We prefer fee which is closest to our ideal. */
+	return fee <= last_fee;
-	old_diff = imaxabs((s64)ideal_fee - (s64)last_fee);
-	new_diff = imaxabs((s64)ideal_fee - (s64)fee);
-
-	/* In case of a tie, prefer new over old: this covers the preference
-	 * for a mutual close over a unilateral one. */
-	log_debug(channel->log, "... That's %s our ideal %"PRIu64,
-		 new_diff < old_diff
-		 ? "closer to"
-		 : new_diff > old_diff
-		 ? "further from"
-		 : "same distance to",
-		 ideal_fee);
-
-	return new_diff <= old_diff;
 }
 
 static void peer_received_closing_signature(struct channel *channel,