Following successive modifications for Shard Split/Shard Merge, the API to used access blockers is confusing. There are some functions duplicated between the registry itself and tenant_migration_access_blocker_util.h. We should do a review of these functions to ensure :

• Minimize the number of different functions
• Ensure the API is coherent (functions similar should be grouped in the registry OR the util but not spread in both)
• Ensure comprehensive unit test coverage (currently spread across tenant_migration_access_blocker_registry_test.cpp and tenant_migration_access_blocker_util_test.cpp)