Make bootstrapping clients wait before trying an authority

If this happens, 0 (or uninitialised data) is used instead of the zeroth schedule item. (This matters most for bridges, which have schedules that start at 60 rather than 0.)

I wonder if any of the existing Tor code does this?

We could set a flag in download_status_t to 1 in download_status_reset, then assert it's set in the low-level functions that take a download_status_t:

• find_dl_schedule_and_len
• download_status_increment_failure
• (not download_status_reset, it sets the flag)
• download_status_get_n_failures
• download_status_get_next_attempt_at