New guard plan - update state whenever we get a new consensus
New guard plan summarized at https://lists.torproject.org/pipermail/tor-dev/2016-July/011234.html
- Update state whenever we get a new consensus (section ON_CONSENSUS)
- Update SAMPLED_GUARDS elements (section SAMPLED)
- Remove obsolete/expired guards
- See entry_guards_compute_status() / remove_obsolete_entry_guards()
- Thoughtworks function: entry_guards_update_profiles()