zpay32: rename to DefaultAssumedFinalCLTVDelta

This renames the paramter to clarify that this value is only used when
no CLTV value was decoded from the payment request

zpay32/invoice.go

@@ -324,7 +324,7 @@ func (invoice *Invoice) MinFinalCLTVExpiry() uint64 {
 		return *invoice.minFinalCLTVExpiry
 	}
 
-	return DefaultFinalCLTVDelta
+	return DefaultAssumedFinalCLTVDelta
 }
 
 // validateInvoice does a sanity check of the provided Invoice, making sure it