event: refresh event info after unregistering

The current code causes a segfault with the following scenario:

 1. Both worker_thread_request_done() event and client_handler() event
    become ready with epoll_wait() in do_event_loop().

 2. do_event_loop() calls worker_thread_request_done(), and it calls
    clear_client_info().  This can happen when the client connection
    is already closed (e.g. in put_request(), tx_main()).

 3. clear_client_info() calls unregister_event(), and it frees the
    client_handler() event.

 3. do_event_loop() tries to call client_handler() but the event is
    already freed and no longer valid.

To fix the problem, this patch calls event_force_refresh() in
unregister_event() and make do_event_loop() call epoll_wait() again.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>