[MAJOR] introduced speculative I/O with epoll()

[MAJOR] introduced speculative I/O with epoll()

The principle behind speculative I/O is to speculatively try to
perform I/O before registering the events in the system. This
considerably reduces the number of calls to epoll_ctl() and
sometimes even epoll_wait(), and manages to increase overall
performance by about 10%.

The new poller has been called "sepoll". It is used by default
on Linux when it works. A corresponding option "nosepoll" and
the command line argument "-ds" allow to disable it.