Implement an in-memory database in CodeSignal, with each round adding more functionality.

Focus on designing a scalable in-memory database that efficiently handles CRUD operations. Start by implementing the basic functionality: create, read, update, and delete records. As you progress through each round, be prepared to add advanced features like indexing, transaction support, or data persistence. Pay attention to performance optimizations and memory management, as these will be critical in demonstrating your ability to build a robust system. Additionally, think about how you would handle concurrent access and potential data integrity issues.