Make consensus expiry tolerance for fallbacks lower when the stale consensus bug is fixed

When legacy/trac#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 legacy/trac#20909 (moved) being fixed, wait until most clients are on 0.3.0 before making this change.

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