trampoline: refactor routes, enable e2e mpp

* Refactor `create_trampoline_route`.
* Enables end-to-end multi-trampoline multipart payments.
  Trampoline-to-legacy payments are still not enabled, as this is
  currently not supported by Eclair.
* Reverts to a global trampoline fee level, as trampoline failures
  are currently not handled properly, see (#7648), which doubles
  fee rates.

electrum/lnpeer.py

@@ -1461,6 +1461,7 @@ class Peer(Logger):
                 invoice_features = payload["invoice_features"]["invoice_features"]
                 invoice_routing_info = payload["invoice_routing_info"]["invoice_routing_info"]
                 # TODO use invoice_routing_info
+                # TODO legacy mpp payment, use total_msat from trampoline onion
             else:
                 self.logger.info('forward_trampoline: end-to-end')
                 invoice_features = LnFeatures.BASIC_MPP_OPT