Trampoline routing.

- trampoline is enabled by default in config, to prevent download of `gossip_db`.
   (if disabled, `gossip_db` will be downloaded, regardless of the existence of channels)
 - if trampoline is enabled:
    - the wallet can only open channels with trampoline nodes
    - already-existing channels with non-trampoline nodes are frozen for sending.
 - there are two types of trampoline payments: legacy and end-to-end (e2e).
 - we decide to perform legacy or e2e based on the invoice:
    - we use trampoline_routing_opt in features to detect Eclair and Phoenix invoices
    - we use trampoline_routing_hints to detect Electrum invoices
 - when trying a legacy payment, we add a second trampoline to the path to preserve privacy.
   (we fall back to a single trampoline if the payment fails for all trampolines)
 - the trampoline list is hardcoded, it will remain so until `trampoline_routing_opt` feature flag is in INIT.
 - there are currently only two nodes in the hardcoded list, it would be nice to have more.
 - similar to Phoenix, we find the fee/cltv by trial-and-error.
    - if there is a second trampoline in the path, we use the same fee for both.
    - the final spec should add fee info in error messages, so we will be able to fine-tune fees

electrum/lnrouter.py

@@ -99,6 +99,16 @@ class RouteEdge(PathEdge):
         features = self.node_features
         return bool(features & LnFeatures.VAR_ONION_REQ or features & LnFeatures.VAR_ONION_OPT)
 
+    def is_trampoline(self):
+        return False
+
+@attr.s
+class TrampolineEdge(RouteEdge):
+    invoice_routing_info = attr.ib(type=bytes, default=None)
+    invoice_features = attr.ib(type=int, default=None)
+    short_channel_id = attr.ib(0)
+    def is_trampoline(self):
+        return True
 
 LNPaymentPath = Sequence[PathEdge]
 LNPaymentRoute = Sequence[RouteEdge]