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/daemon.py

@@ -450,7 +450,8 @@ class Daemon(Logger):
         if self.network:
             self.network.start(jobs=[self.fx.run])
             # prepare lightning functionality, also load channel db early
-            self.network.init_channel_db()
+            if self.config.get('use_gossip', False):
+                self.network.start_gossip()
 
         self.taskgroup = TaskGroup()
         asyncio.run_coroutine_threadsafe(self._run(jobs=daemon_jobs), self.asyncio_loop)