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/base_wizard.py

@@ -545,6 +545,8 @@ class BaseWizard(Logger):
 
     def create_keystore(self, seed, passphrase):
         k = keystore.from_seed(seed, passphrase, self.wallet_type == 'multisig')
+        if self.wallet_type == 'standard' and self.seed_type == 'segwit':
+            self.data['lightning_xprv'] = k.get_lightning_xprv(None)
         self.on_keystore(k)
 
     def on_bip43(self, seed, passphrase, derivation, script_type):