zincati 0.0.9

Changes:

• zincati: rework to use std::future
• zincati: add completion timeout to HTTP requests
• metrics: stabilize metrics endpoint
• update-agent: rework steady/polling state
• docs: add state-machine diagram
• cargo: relax dependencies micro versions
• cargo: update all dependencies

Tracker: https://github.com/coreos/zincati/milestone/6