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

Introduced a NullLoggerFactory which aligns more cleanly with the dependency requirements of the components

Declared content length for checksums to avoid chunked transfer encoding which is troublesome with nginx (HTTP 411)