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/network.py

@@ -359,22 +359,25 @@ class Network(Logger, NetworkRetryManager[ServerAddr]):
     def has_channel_db(self):
         return self.channel_db is not None
 
-    def init_channel_db(self):
-        if self.channel_db is None:
-            from . import lnrouter
-            from . import channel_db
+    def start_gossip(self):
+        from . import lnrouter
+        from . import channel_db
+        from . import lnworker
+        if not self.config.get('use_gossip'):
+            return
+        if self.lngossip is None:
             self.channel_db = channel_db.ChannelDB(self)
             self.path_finder = lnrouter.LNPathFinder(self.channel_db)
             self.channel_db.load_data()
-
-    def start_gossip(self):
-        if self.lngossip is None:
-            from . import lnworker
             self.lngossip = lnworker.LNGossip()
             self.lngossip.start_network(self)
 
     def stop_gossip(self):
-        self.lngossip.stop()
+        if self.lngossip:
+            self.lngossip.stop()
+            self.lngossip = None
+            self.channel_db.stop()
+            self.channel_db = None
 
     def run_from_another_thread(self, coro, *, timeout=None):
         assert self._loop_thread != threading.current_thread(), 'must not be called from network thread'