Fix fd leak when timerfd is disabled on Linux.

Hello.

I tried to use jubatus in CentOS 5.8 but jubatus did not run normally.
When I investigated, there was the file-descriptor leak in timer of mpio with --disable-timerfd configure option on Linux.

With --disable-timerfd on Linux, timer of mpio uses a pair of pipes for communication between timer handler and epoll.  And timer handler side of the pipes is not closed.

This request fixes this leak and adds test code(test/many_timer.cc).