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
- makefile: Don't refresh deps automatically, causes double build
- storage: Move GetMessage, GetMessages (Mailbox), PurgeMessages to the
Store API for #69
- storage: Remove Mailbox.Name method for #69
- test: Create new test package for #79
- test: Implement StoreStub, migrate some tests off MockDataStore for
task #80
- rest & webui: update controllers to use new Store methods
- All packages go into either cmd or pkg directories
- Most packages renamed
- Server packages moved into pkg/server
- sanitize moved into webui, as that's the only place it's used
- filestore moved into pkg/storage/file
- Makefile updated, and PKG variable use fixed