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/sql_db.py

@@ -24,6 +24,7 @@ class SqlDB(Logger):
     def __init__(self, asyncio_loop: asyncio.BaseEventLoop, path, commit_interval=None):
         Logger.__init__(self)
         self.asyncio_loop = asyncio_loop
+        self.stopping = False
         self.path = path
         test_read_write_permissions(path)
         self.commit_interval = commit_interval
@@ -31,6 +32,9 @@ class SqlDB(Logger):
         self.sql_thread = threading.Thread(target=self.run_sql)
         self.sql_thread.start()
 
+    def stop(self):
+        self.stopping = True
+
     def filesize(self):
         return os.stat(self.path).st_size
 
@@ -40,7 +44,7 @@ class SqlDB(Logger):
         self.logger.info("Creating database")
         self.create_database()
         i = 0
-        while self.asyncio_loop.is_running():
+        while not self.stopping and self.asyncio_loop.is_running():
             try:
                 future, func, args, kwargs = self.db_requests.get(timeout=0.1)
             except queue.Empty: