lnworker: improve route creation

- Separates the trampoline and local routing multi-part payment cases.
- Ask only for splits that don't send over a single channel (those have
  been tried already in the single-part case).
- Makes sure that create_routes_for_payment only yields partial routes
  that belong to a single split configuration.
- Tracks trampoline fee levels on a per node basis, previously, in the
  case of having two channels with a trampoline forwarder, the global
  fee level would have increased by two levels upon first try.

electrum/tests/test_mpp_split.py

@@ -122,3 +122,7 @@ class TestMppSplit(ElectrumTestCase):
             mpp_split.PART_PENALTY = 0.3
             splits = mpp_split.suggest_splits(1_000_000_000, channels_with_funds, exclude_single_part_payments=False)
             self.assertEqual(3, len(splits[0].config[(0, 0)]))
+        with self.subTest(msg="exclude all single channel splits"):
+            mpp_split.PART_PENALTY = 0.3
+            splits = mpp_split.suggest_splits(1_000_000_000, channels_with_funds, exclude_single_channel_splits=True)
+            self.assertEqual(1, len(splits[0].config[(0, 0)]))