Configuration bug fixes, startup reliability, and iterator improvements

[nagendramishr]

• Bug Fix: Start App Insights with app config setting
• Bug Fix: environment defaults and BackendOptions were getting mixed-up
• Bug Fix: Probes not getting logged
• Bug Fix: Environment variables not being defaulted to
• Bug Fix: Make sure program does not exit until shutdown completes
• Make shared iterator use either single Pass or maxAttempts, default to single Pass.
• Don't invalidate shared iterators unless using latency load balancer and the order changed
• In the case of a 404, use the same response path as a normal response
• Bug fix: possible loss of event in edge case
• Don't report ready until: workers, backends, profiles are all ready.