EntryGuardDownSince/UnreachableSince state lines are unnecessary?
When we first added the guard relay design, we needed to discard guards that hadn't been listed for a while, so they didn't clutter up our state file forever. We chose ENTRY_GUARD_REMOVE_AFTER (302460*60), and we do the deed in remove_dead_entry_guards().
But later we realized that for load balancing purposes we need to discard guards after a month or two whether they've been down or not. See date_is_bad in remove_obsolete_entry_guards().
I suspect the latter is good enough, and we can get rid of the former logic. I think that might even include getting rid of the lines in the state file, since we no longer need to track long-term downness of guards.