Timed Key-Value Store

Implement a key-value store that supports the following operations:

1. set(key, value, ttl): Stores the key with the given value and a time-to-live (TTL) in seconds. The key should automatically expire and no longer be available after the TTL has elapsed.

2. get(key): Returns the value associated with the key if it has not expired; otherwise, it should return a null value or an appropriate indicator that the key does not exist.

Additional Requirements:

• If an existing key is set again with a new TTL, it should update its expiration time accordingly.
• The system should efficiently handle expired keys. When a key is checked by get(), ensure that it is not returned if expired.
• Consider the possibility of concurrent calls to set() and get() and design your solution to be efficient in terms of both time and space.
• You may simulate or use a system clock to obtain the current time for managing TTL.

Your solution should be implemented in the programming language of your choice. Please ensure that the code is well-structured and includes comments where appropriate.