Optimize descriptor registration in event loop

[EdmondDantes]

Refactor the event loop to register descriptors only once during their lifetime.
Instead of repeatedly adding and removing descriptors on each await operation, a descriptor will remain registered in the loop until it is closed.

This approach minimizes the number of syscalls (epoll_ctl ADD/DEL) by relying on a one-time registration and deferred removal.
It reduces overhead in high-concurrency scenarios and aligns the implementation with common practices in event loop libraries (e.g., libuv, Swoole).