event: when unreffing an event source from its own handler, detach fd from epoll

event: when unreffing an event source from its own handler, detach fd from epoll

The pattern of unreffing an IO event source and then closing its fd is
frequently seen in even source callbacks. Previously this likely
resultet in us removing the fd from the epoll after it was closed which
is problematic, since while we were dispatching we always kept an extra
reference to event source objects because we might still need it later.