closingd: use a more accurate fee for closing fee negotiation.

We were actually using the last commit tx's size, since we were
setting it in lightningd.  Instead, hand the min and desired feerates
to closingd, and (as it knows the weight of the closing tx), and have
it start negotiation from there.

This can be significantly less when anchor outputs are enabled: for
example in test_closing.py, the commit tx weight is 1124 Sipa, the
close is 672 Sipa!

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Changed: Protocol: Use a more accurate fee for mutual close negotiation.

tests/utils.py

@@ -151,6 +151,12 @@ def basic_fee(feerate):
     return (weight * feerate) // 1000
 
 
+def closing_fee(feerate, num_outputs):
+    assert num_outputs == 1 or num_outputs == 2
+    weight = 424 + 124 * num_outputs
+    return (weight * feerate) // 1000
+
+
 def scriptpubkey_addr(scriptpubkey):
     if 'addresses' in scriptpubkey:
         return scriptpubkey['addresses'][0]