backups: restore from closing tx, sweep to_remote

* add a method for backups to sweep to_remote
* to_remote sweeping needs the payment_basepoint's private key
  to sign the sweep transaction
* we restore the private key from our funding multisig pubkey
  (pubished with the closing transaction) and a static payment key secret

electrum/lnchannel.py

@@ -56,9 +56,10 @@ from .lnutil import (Outpoint, LocalConfig, RemoteConfig, Keypair, OnlyPubkeyKey
                      ShortChannelID, map_htlcs_to_ctx_output_idxs,
                      fee_for_htlc_output, offered_htlc_trim_threshold_sat,
                      received_htlc_trim_threshold_sat, make_commitment_output_to_remote_address, FIXED_ANCHOR_SAT,
-                     ChannelType, LNProtocolWarning)
+                     ChannelType, LNProtocolWarning, ctx_has_anchors)
 from .lnsweep import create_sweeptxs_for_our_ctx, create_sweeptxs_for_their_ctx
 from .lnsweep import create_sweeptx_for_their_revoked_htlc, SweepInfo
+from .lnsweep import create_sweeptx_their_backup_ctx
 from .lnhtlc import HTLCManager
 from .lnmsg import encode_msg, decode_msg
 from .address_synchronizer import TX_HEIGHT_LOCAL
@@ -594,14 +595,19 @@ class ChannelBackup(AbstractChannel):
         return True
 
     def create_sweeptxs_for_their_ctx(self, ctx):
-        return {}
+        return create_sweeptx_their_backup_ctx(chan=self, ctx=ctx, sweep_address=self.get_sweep_address())
 
     def create_sweeptxs_for_our_ctx(self, ctx):
         if self.is_imported:
             return create_sweeptxs_for_our_ctx(chan=self, ctx=ctx, sweep_address=self.get_sweep_address())
         else:
-            # backup from op_return
-            return {}
+            return
+
+    def maybe_sweep_revoked_htlc(self, ctx: Transaction, htlc_tx: Transaction) -> Optional[SweepInfo]:
+        return None
+
+    def extract_preimage_from_htlc_txin(self, txin: TxInput) -> None:
+        return None
 
     def get_funding_address(self):
         return self.cb.funding_address