Key-Value Store with TTL

Implement an in-memory key-value store that supports the following operations:

• set(key, value, ttl): Stores the key-value pair in the store. The ttl parameter specifies the time, in seconds, after which the key should expire and be removed from the store. If ttl is not provided, the key should persist indefinitely.

• get(key): Retrieves the current value for the given key if it exists and has not expired; otherwise, returns an appropriate value (e.g., null or similar) indicating that the key is unavailable.

• count(): Returns the total number of keys currently stored (only counting keys that have not expired).

Additional requirements:

1. Expired keys must be cleaned up automatically without requiring explicit calls to get() or external triggers.
2. The solution should be designed with efficiency in mind, and it should ideally support concurrent operations from multiple threads (if applicable in your language).
3. Consider handling edge cases, such as updating the ttl for an existing key or dealing with a set operation on a key that already exists.
4. You are free to choose any programming language and use the built-in data structures available in that language, but avoid using third-party libraries for the core logic.

Design and implement this key-value store. Ensure your solution is well-tested and documented.