Sample SAMPLED_GUARDS from GUARDS per new guard plan
New guard plan summarized at https://lists.torproject.org/pipermail/tor-dev/2016-July/011234.html
- Sample SAMPLED_GUARDS from GUARDS
- Fill in state for each sampled guard (added_on_date, etc.)
- Save/load SAMPLED_GUARDS to/from state
- Unittests for sampling/saving/loading
- Thoughtworks function: fill_sampled_guards_from_entrynodes()
- Thoughtworks unittest: test_fill_in_sampled_set()