Reduce the exponential backoff variance

When we increased the exponent to compensate for there being no initial delay on schedules, this also increased the schedule variance.

We could reduce this variance by having a smaller range of multipliers, for example, choosing next delay from [delay*2, delay*3], rather than [1, delay*3].

We could also add a linear minimum delay increase, for example, choosing next delay from [10, delay*3], rather than [1, delay*3].

Follow-up to #20499 (moved), #20597 (moved), and #20534 (moved).