Postpone fetches based on should_delay_dir_fetch(), not DisableNetwork

Without this fix, when running with bridges, we would try fetching
directory info far too early, and have up to a 60 second delay if we
started with bridge descriptors available.

Fixes bug 11965. Fix on 0.2.3.6-alpha, arma thinks.
Merge branch 'bug11965_v2'