Exclude relays that have been down for 1 or more days from the fallback
candidate list.
When a relay operator has multiple relays, this prioritises relays that are
up over relays that are down.
Fixes issue #20926.
7 days is a tradeoff between the expected time between major Tor releases,
which is 6 months, and the number of relays with enough stability.
Relays whose OnionOO stability timer is reset on restart by bug #18050
should upgrade to Tor 0.2.8.7 or later, which has a fix for this issue.
Closes ticket #20880; maintains short-term fix in e220214 in tor-0.2.8.2-alpha.