epoll: Use epoll queues to speedup multiple duped fds

When we are dumping epoll and one of target fd is been
duped we can reuse already collected fds rbtree to find
proper target. We handle it in a lazy way:

 - try use plain regular bsearch first, in case of all
   targets are not duped we checkpoint epoll immediately

 - if bsearch failed we put this epoll entry into a queue
   and run its dumping later when all other files in the
   process are already dumped. At this moment fds tree
   should already has all target files in rbtree thus
   we can simply lookup for it

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>