Mapping Service Bug Fixes

[JeremyCaney]

A deep audit of the existing logic identified a number of bugs related to the topic mapping services. These are grouped together here for serial resolution.

Tasks

• Deleted nested topics aren't just removed from memory, but persisted (ReverseTopicMappingService.PopulateTargetCollectionAsync())
• New nested children need their Parent set correctly (ReverseTopicMappingService.PopulateTargetCollectionAsync())
• Relationships should be validated before the existing relationship set is cleared, so a miss doesn't wipe good data (ReverseTopicMappingService.SetRelationships)
• Mapping to a mismatched content type should throw a typed MappingModelValidationException instead of a raw cast failure (ReverseTopicMappingService.MapAsync<T>)
• Update the navigation menu cache key so requests with different depth, payload settings don't collide (CachedHierarchicalTopicMappingService<T>)
• Create unit tests to validate the above logic