Enhance S3 FIFO policy with atomic frequency management and shared access

- Introduced `IntoIter` for consuming cache entries, improving iteration efficiency.
- Updated `Node` structure to use `AtomicU8` for frequency, allowing concurrent readers to increment frequency without locking.
- Added `get_shared` method for concurrent access, enabling frequency updates in a lock-free manner.
- Refactored `get` and `get_mut` methods to handle misses more gracefully and maintain metrics accurately.
- Improved documentation for methods and added panic conditions for better user guidance.

Author: TKorr