Maintain set of PRIMARY_GUARDS per new guard plan
New guard plan summarized at https://lists.torproject.org/pipermail/tor-dev/2016-July/011234.html
- Maintain set of PRIMARY_GUARDS
- Functions to extract set of (live) primary guards out of FILTERED_GUARDS
- Unittest
- Thoughtworks function: retry_primary_guards() / next_primary_guard()
- Thoughtworks unittests: test_next_primary_guard()