ContentMerger now has to take a TopicMergeStrategy
ContentMerger now merges topics!

- Strategies include:
  - Throw away new ones
  - Throw away old ones
  - Replace based on a given equivalence
KM data handler can just replace all topic refs
KM data handler no longer needs to resolve topics

This was originally needed for a check I added to fix an earlier data mistake.
Getty ingest now uses ContentMerger's topic merging
Things have to give ContentMerger a TopicsMergeStrategy