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)

Simplified VersionConstraint.getRanges() to getRange(), an aggregation of ranges is again a range and the semantics of the aggregation are an impl detail