New flow for submarine swaps:

- client requests payment_hash from the server
 - client sends an invoice with that hash
 - client waits to receive HTLCs, then broadcasts funding tx

This means that we now use same script for normal and reverse swaps.
The new flow is enabled by setting option LIGHTNING_SWAP_HTLC_FIRST
in the client. The old protocol is still supported server-side.

electrum/simple_config.py

@@ -890,6 +890,7 @@ class SimpleConfig(Logger):
     LIGHTNING_USE_GOSSIP = ConfigVar('use_gossip', default=False, type_=bool)
     LIGHTNING_USE_RECOVERABLE_CHANNELS = ConfigVar('use_recoverable_channels', default=True, type_=bool)
     LIGHTNING_ALLOW_INSTANT_SWAPS = ConfigVar('allow_instant_swaps', default=False, type_=bool)
+    LIGHTNING_SWAP_HTLC_FIRST = ConfigVar('swap_htlc_first', default=False, type_=bool)
     LIGHTNING_TO_SELF_DELAY_CSV = ConfigVar('lightning_to_self_delay', default=7 * 144, type_=int)
     LIGHTNING_MAX_FUNDING_SAT = ConfigVar('lightning_max_funding_sat', default=LN_MAX_FUNDING_SAT_LEGACY, type_=int)