Move guard state out of globals per new guard plan

New guard plan summarized here at https://lists.torproject.org/pipermail/tor-dev/2016-July/011234.html

1. Create main guard selection state data structure (instead of using global variables)

• Ideally there should be no global variables, so that in the future we can have multiple parallel guardsets (e.g. prop247)
• Thoughtworks: guard_selection_t