lnwallet: fix receiver htlc script

* Fixes a bug in script_utils.go. CSV instead of CLTV was being used
  within the timeout clause of the receivers HTLC (sender can reclaim
the HTLC).

lnwallet/script_utils.go

@@ -192,7 +192,7 @@ func receiverHTLCScript(absoluteTimeout, relativeTimeout uint32, senderKey,
 	// indefinately.
 	builder.AddOp(txscript.OP_NOTIF)
 	builder.AddInt64(int64(absoluteTimeout))
-	builder.AddOp(OP_CHECKSEQUENCEVERIFY)
+	builder.AddOp(txscript.OP_CHECKLOCKTIMEVERIFY)
 	builder.AddOp(txscript.OP_DROP)
 	builder.AddOp(txscript.OP_ENDIF)