add epoll set size tracking

[egnchen]

avoid calling epoll_wait if epoll fd set is empty and timeout == 0
NOTE: we still calls epoll_wait when timeout != 0 since per POSIX standard the epoll_wait will "block until some file descriptor is later added to the interest list (in another thread) and that file descriptor becomes ready."(https://man7.org/linux/man-pages/man2/epoll_wait.2.html)