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/bitcoin.py

@@ -295,6 +295,10 @@ def push_script(data: str) -> str:
     return _op_push(data_len) + bh2u(data)
 
 
+def make_op_return(x:bytes) -> bytes:
+    return bytes([opcodes.OP_RETURN]) + bytes.fromhex(push_script(x.hex()))
+
+
 def add_number_to_script(i: int) -> bytes:
     return bfh(push_script(script_num_to_hex(i)))