Update dir-spec with client fallback directory mirror attempt and timeout behaviour
Let's add these lines:
Clients try several fallback directory mirrors, and use the first one that connects. Each attempt happens after a short delay, regardless of the state of the previous attempt, until at least one attempt has connected.
When several fallback directory mirrors have failed, clients start trying directory authorities in a similar fashion.
I don't think we designed any explicit timeout behaviour, so we are probably using whatever was there before 0.2.8.