Cache Concurrency Guard (Associations)

[JeremyCaney]

A follow-up to #117: When two web requests try to resolve the same topic's relationships or references at the same time, both attempt to wire up the results to the same shared in-memory topic, including the other topic's IncomingRelationships list, which can corrupt that shared state or throw an exception. This closes that race with a lock, scoped narrowly enough to avoid deadlocking on the batch-load work (e.g., #127).

Tasks

• Add a unit test that triggers the race (i.e., two concurrent reads resolving the same relationships)
• Wrap the wire-up steps in CachedTopicRepository under the existing cache-wide lock
  • The batch-load step itself stays unlocked, since locking it could deadlock for bidirectional references
• Confirm un-cached repositories (TopicRepository, SqlTopicRepository) are unaffected (they shouldn't be)
• Create unit tests to validate the above logic