Design and implement an in-memory key-value store that supports set, transactional begin, commit, and abort operations.

To design an in-memory key-value store with transactional capabilities, we need to consider concurrency control and memory management. A naive approach involves using a hash map protected by a read-write lock. To improve performance and reduce lock contention, we can shard the hash map into multiple segments, allowing concurrent access to different segments. For transactional operations (begin, commit, abort), each writer can be assigned a local buffer to store changes. During a commit, these changes are merged into the main store. An abort would simply discard the local buffer. To optimize memory, we should ensure that only the latest committed values are retained, and any intermediate or stale entries in transaction buffers are properly cleaned up.