When a client hasn't used the Tor network for a few days, its consensus expires. If the authorities are blocked while it's offline, it can't bootstrap. However, it has (almost) enough information to bootstrap from a semi-trusted relay - its guard. This change requires: * a proposal * an additional line `GuardFallbackDir` in the state file, formatted like a FallbackDir line, if the guard is also a directory mirror When legacy/trac#12538 is implemented, every relay (and therefore every Guard) will be a directory cache by default.