shard merge has started polluting TenantMigrationAccessBlockerRegistry and made the API a bit confusing. TenantMigrationAccessBlockerRegistry should expose APIs that are generic to donor and recipient. And, tenant_migration_access_blocker_util.cpp will have (if actually needed... we should evaluate how useful these specific util methods actually are) recipient or donor specific methods, like getTenantMigrationRecipientAccessBlocker().

Additionally, we should clean up the singleton shard merge donor access blocker (donorAccessBlocker) in favor of using the existing map of tenant id-to-access blocker with some sort of special DONOR_ key.