Use older consensuses when new ones are not available.

See the use of NETWORKSTATUS_ALLOW_SKEW and REASONABLY_LIVE_TIME in the C tor implementation: It is a good idea to keep running with an older version of the consensus when a new one isn't available.

This makes clients more robust in the case that the authorities have an outage.

These values are 24 hours in Tor; we should consider raising them to 48 or 72 hours. (Ask the network health team.)