There are two major benefits to making a new ShardSplitAccessBlocker:

• Log messages using TenantMigrationDonorAccessBlocker can be confusing when debugging a split operation, since messages relating to a "tenant migration" begin appearing from nowhere
• Shard split allows splitting multiple tenants at the same time, so some logic could be deduplicated especially where we loop on a list of tenant ids in multiple parts of the codebase.