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/tests/test_wallet.py

@@ -167,7 +167,7 @@ class TestCreateRestoreWallet(WalletTestCase):
         wallet = d['wallet']  # type: Standard_Wallet
 
         # lightning initialization
-        self.assertTrue(wallet.db.get('lightning_privkey2').startswith('xprv'))
+        self.assertTrue(wallet.db.get('lightning_xprv').startswith('zprv'))
 
         wallet.check_password(password)
         self.assertEqual(passphrase, wallet.keystore.get_passphrase(password))