Refactor payment forwarding:

- all forwarding types use the same flow
 - forwarding callback returns a htlc_key or None
 - forwarding info is persisted in lnworker:
   - ongoing_forwardings
   - downstream to upstream htlc_key
   - htlc_key -> error_bytes

electrum/lnutil.py

@@ -66,6 +66,13 @@ hex_to_bytes = lambda v: v if isinstance(v, bytes) else bytes.fromhex(v) if v is
 json_to_keypair = lambda v: v if isinstance(v, OnlyPubkeyKeypair) else Keypair(**v) if len(v)==2 else OnlyPubkeyKeypair(**v)
 
 
+def serialize_htlc_key(scid:bytes, htlc_id: int):
+    return scid.hex() + ':%d'%htlc_id
+
+def deserialize_htlc_key(htlc_key:str):
+    scid, htlc_id = htlc_key.split(':')
+    return bytes.fromhex(scid), int(htlc_id)
+
 @attr.s
 class OnlyPubkeyKeypair(StoredObject):
     pubkey = attr.ib(type=bytes, converter=hex_to_bytes)