The TenantMigrationDonorService sometimes uses the _stateDoc as a source of truth for the current state, and other times uses _durableState.state. Additionally, we sometimes save a copy of the in-memory state document off in order to update the durable state document which has lead to data races (SERVER-61565). We should reconsider how we track state to eliminate this class of error, and to make the code more readable for future maintainers.