New guard plan - guard selection for circuits
New guard plan summarized at https://lists.torproject.org/pipermail/tor-dev/2016-July/011234.html
- Selecting guards for circuits
- Meant to replace choose_random_entry_impl()
- See section SELECTING in prop271
- Add new circuit states to or_circuit_t
- Implement the guard selection logic
- Unittests on circuit state machine
- Unittests on guard selection logic