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/keystore.py

@@ -613,6 +613,11 @@ class BIP32_KeyStore(Xpub, Deterministic_KeyStore):
         cK = ecc.ECPrivkey(k).get_public_key_bytes()
         return cK, k
 
+    def get_lightning_xprv(self, password):
+        xprv = self.get_master_private_key(password)
+        rootnode = BIP32Node.from_xkey(xprv)
+        node = rootnode.subkey_at_private_derivation("m/67'/")
+        return node.to_xprv()
 
 class Old_KeyStore(MasterPublicKeyMixin, Deterministic_KeyStore):