Support lazy-loading in `CachedTopicRepository`

[JeremyCaney]

Instead of eagerly loading the entire topic tree, as we currently do, instead update the CachedTopicRepository to support dynamically loading children, relationships, and potentially even extended attributes as requested.

Notes

• This will benefit from extended the IsFullyLoaded properties into a LoadState enum with e.g., None, Fully, and Partially
• This will require support in SqlTopicRepository:
  • When a topic's RangeLeft-RangeRight is 1, it's Children collection's LoadState can be set to Fully
  • Even if the target relationships aren't in the Load() or the cache, we can use their presence and matches to set LoadState for the relationships
    • This won't be possible for IncomingRelationships
• Ideally, we should support loading a "Skeleton", which includes all topics, all relationships, and all indexed attributes, but not the data- and memory-intensive extended attributes
• We will still need the ability via e.g. the isRecursive argument to deep load an entire tree
  • This should likely be the standard for the Root:Configuration section, since it has a lot of internal cross-references
    • This standard should be established in the SampleActivator, not in the repositories themselves