Avoid constant initial download delays in download schedules
When a download schedule has a non-zero initial delay, the delay is used as-is. We use this to delay the initial authority consensus download (so fallbacks get used), and bridge descriptor re-downloads.
But these constant delays produce an obvious traffic pattern, so we should work out how to randomise them. Using a random value from [delay/2, delay*3/2] would be a good start.