Refactor server life-cycle into it's own file, make service startup and monitoring more consistent and testable.
* Extract services creation in preparation for DI
* pop3: rename New to NewServer
* lifecycle: Add fatal error Notify()
* web: Introduce Server struct w/ Notify()
* Extract Start in lifecycle
* Add Start() to Hub
* RetentionScanner startup consistent with other svcs
* Remove global shutdown channel
* Implement a readiness notification system
