Maintain CONFIRMED_GUARDS per new guard plan
New guard plan summarized at https://lists.torproject.org/pipermail/tor-dev/2016-July/011234.html
- Maintain CONFIRMED_GUARDS
- Add guard to CONFIRMED_GUARDS when circuit succeeds
- Fill in state info for each confirmed guard (confirmed_on_date, etc.)
- Migration from old state format to new
- Thoughtworks unittest: test_used_guards_parse_state()
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