There is no point in first checking for the expired timelocks and
then constructing a `select!` that also watches for the timelock to
expiry.
We can simply only have the select! invocation to achieve the same
effect. In case the timelock is already expired, this future will
resolve immediately.
Normally, the polling order of `select!` is pseudo-random. We
configure it to be _biased_ here to make sure the futures are polled
in order.