Deterministic NodeID:

- use_recoverable_channel is a user setting, available
   only in standard wallets with a 'segwit' seed_type
 - if enabled, 'lightning_xprv' is derived from seed
 - otherwise, wallets use the existing 'lightning_privkey2'

Recoverable channels:
 - channel recovery data is added funding tx using an OP_RETURN
 - recovery data = 4 magic bytes + node id[0:16]
 - recovery data is chacha20 encrypted using funding_address as nonce.
   (this will allow to fund multiple channels in the same tx)

GUI:
  - whether channels are recoverable is shown in wallet info dialog.
  - if the wallet can have recoverable channels but has an old node_id,
    users are told to close their channels and restore from seed
    to have that feature.

electrum/channel_db.py

@@ -867,6 +867,13 @@ class ChannelDB(SqlDB):
         with self.lock:
             return self._policies.copy()
 
+    def get_node_by_prefix(self, prefix):
+        with self.lock:
+            for k in self._addresses.keys():
+                if k.startswith(prefix):
+                    return k
+        raise Exception('node not found')
+
     def to_dict(self) -> dict:
         """ Generates a graph representation in terms of a dictionary.