Tweaked configuration for maven-bundle-plugin to produce Bundle-Name header confirming with incubation requirements

Added missing optional markers for aether-connector-asynchttpclient bundle

Moved DefaultRepositorySystemSession, DefaultRepositoryCache and DefaultSessionData over into API module to improve usuability

- Made get*Properties() provide read-only views
- Made copy constructor clone incoming maps to avoid unintended data coupling with input session

Refactored DefaultServiceLocator to use composition rather than inheritance to support error handling hook