CAUSEWAY-4044: Work towards immutable ObjectSpecification - #3697

Motivation: The current ObjectSpecification implementation is fragile and leads to a non deterministic outcome of the Metamodel inspection.

• Factor out Member creation into factory classes
• Facet caching is in conflict with lazy initialization. Query for Facets that are not populated yet can taint the cache and lead to an incorrect Metamodel.
• When evicting a Spec from the Spec Loader's cache, there are no guarantees that no other Spec is holding on the the eviced Spec.
• Unclear policy, as to whether we need to lookup the type hierarchy for Facets. (see Hierarchical)
• Mixins may be discovered only after the prior known set of mixins had already been added to object specifications.
• To yield immutability perhaps use the Builder design pattern. (namely an ObjectSpecificationBuilder)

Task-Url: https://issues.apache.org/jira/browse/CAUSEWAY-4044