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

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