Retry schedule for guards per new guard plan
New guard plan summarized at https://lists.torproject.org/pipermail/tor-dev/2016-July/011234.html
- Retry schedule for guards (section RETRYING)
- See entry_is_time_to_retry()
- Unittest
New guard plan summarized at https://lists.torproject.org/pipermail/tor-dev/2016-July/011234.html