Only consider pending_timeout_sec if it is set

[ok300]

This PR uses tokio::select with a conditional tokio::sleep, which allows us to conditionally sleep.

If pending_timeout_sec is set, the first of send_payment, sleep will return. So if the timeout occurs first, the method will return.

If pending_timeout_sec is not set, it will only execute send_payment, because the second branch of tokio::select is ignored.

[dangeross]

I think this can be closed because tokio::select doesn't fulfil the intension of completing the payment branch timeout or not.

[ok300]

Correct. I forgot to close this as I was iterating through fixes.

Superseded by #958 .