Make consensus expiry tolerance for fallbacks lower when the stale consensus bug is fixed
When #20909 (moved) is fixed, make CONSENSUS_EXPIRY_TOLERANCE in scripts/maint/updateFallbackDirs.py lower than 24 hours:
- In update_consensus_networkstatus_fetch_time_impl, directory mirrors download consensuses between 2 and 90 minutes after they are created. So the tolerance can be as low as -90.
- In networkstatus_get_reasonably_live_consensus, REASONABLY_LIVE_TIME is 24 hours after consensus expiry, so the tolerance must not be higher than 24 hours.
- Relay clocks are allowed to drift 12 hours ahead, and 24 hours behind. So it might be reasonable to have the tolerance be 0, or a slightly positive value.
If 0.2.9 is released without #20909 (moved) being fixed, wait until most clients are on 0.3.0 before making this change.