CC-5709: Airtime Analyzer

* Retry HTTP requests (kinda crappy, should find a better design pattern to
  solve this) - CC-5775
* Persist failed HTTP requests at shutdown
Merge branch 'cc-5709-airtime-analyzer' of github.com:sourcefabric/Airtime into cc-5709-airtime-analyzer
Merge branch 'cc-5709-airtime-analyzer' into cc-5709-airtime-analyzer-saas