askrene: implement reduce_num_flows in refine, using increase_flows().

Now we simply call it at the end.  We need to check it hasn't violated fee maxima, but
otherwise it's simple.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Fixed: Plugins: `askrene` now handles limits on number of htlcs much more gracefully.

tests/test_askrene.py

@@ -1782,8 +1782,7 @@ def test_simple_dummy_channel(node_factory):
 
 def test_maxparts_infloop(node_factory, bitcoind):
     # Three paths from l1 -> l5.
-    # FIXME: enhance explain_failure!
-    l1, l2, l3, l4, l5 = node_factory.get_nodes(5, opts=[{'broken_log': 'plugin-cln-askrene.*the obvious route'}] + [{}] * 4)
+    l1, l2, l3, l4, l5 = node_factory.get_nodes(5)
 
     for intermediate in (l2, l3, l4):
         node_factory.join_nodes([l1, intermediate, l5])
@@ -1805,16 +1804,14 @@ def test_maxparts_infloop(node_factory, bitcoind):
                              final_cltv=5)
     assert len(route['routes']) == 3
 
-    # Now with maxparts == 2.  Usually askrene can't figure out why it failed,
-    # but sometimes it gets a theory.
-    with pytest.raises(RpcError):
-        l1.rpc.getroutes(source=l1.info['id'],
-                         destination=l5.info['id'],
-                         amount_msat=amount,
-                         layers=[],
-                         maxfee_msat=amount,
-                         final_cltv=5,
-                         maxparts=2)
+    route = l1.rpc.getroutes(source=l1.info['id'],
+                             destination=l5.info['id'],
+                             amount_msat=amount,
+                             layers=[],
+                             maxfee_msat=amount,
+                             final_cltv=5,
+                             maxparts=2)
+    assert len(route['routes']) == 2
 
 
 def test_askrene_timeout(node_factory, bitcoind):