Derive FILTERED_GUARDS / USABLE_FILTERED_GUARDS from SAMPLED_GUARDS per new guard plan
New guard plan summarized at https://lists.torproject.org/pipermail/tor-dev/2016-July/011234.html
- Derive FILTERED_GUARDS / USABLE_FILTERED_GUARDS from SAMPLED_GUARDS
- The filtering function can be called at any point (e.g. ON_CONSENSUS)
- Write unittests ensuring correctness of filtering
- See populate_live_entry_guards()
- Thoughtworks function: filter_set() / filter_sampled()