feat: implement TTL support

[AlexJuca]

This PR implements #6

Adds TTL support with four new commands: DEL, EXPIRE, TTL, and PERSIST. Key expiration uses a hybrid approach lazy checks on every read/write operation plus an active sweep via a 100ms event loop timer (kqueue, epoll, and io_uring).

SET clears any existing TTL, matching Redis semantics most users would expect.

Includes 34 integration tests covering the full lifecycle: expiry, persistence, edge cases like EXPIRE 0, double DEL, and lazy expiry across all arithmetic commands.

Summary of supposed commands:

Command	Usage	Description
SET	SET key value [EX seconds]	Store a key-value pair, optionally setting a TTL atomically

Expiration (TTL)

Command	Usage	Description
EXPIRE	EXPIRE key seconds	Set a timeout on a key (in seconds)
TTL	TTL key	Get the remaining time-to-live of a key (-1 = no TTL, -2 = key missing)
PERSIST	PERSIST key	Remove the timeout from a key