channel padding: Permit reuse of random number of unexpired timer

Rationale:

We want to avoid doing any per-cell work that can be avoided. Generating random numbers is such work.

The effect of this is to slightly rebalance the padding distribution towards lower padding density, compared to the other way of doing things, since any numbers we reuse have, by definition, not yet expired from any earlier inactivity.

The code I have in Arti only selects a new random number of the previous one was "used", by the timer firing.