Fix fetch certs interval

[sea-snake]

As observed by @peterpeterparker, earlier retry delays only applied to the first outcall error instead of any failed future outcalls.

Thanks @peterpeterparker for catching this one.

Changes

• Switch around meaning of delay argument in schedule_fetch_certs, None now default to FETCH_CERTS_INTERVAL instead of 0.
• When outcall succeeds, schedule_fetch_certs is called with None:
  • Will default to FETCH_CERTS_INTERVAL with timer.
  • Will use 60 * 2 as retry delay.
• When outcall fails, schedule_fetch_certs is called with Some:
  • Will use delay with timer.
  • Will use min(FETCH_CERTS_INTERVAL, max(delay, 60) * 2) as retry delay.

Tests