Prefetch pages by madvise(2) to prevent random accesses from threads.

When page cache is cleared, it did not scale at all even on SSD machine.
The SSD's random access speed is 75MB/sec, while HDD has almost the same random access speed.

I guessed it was because physical disk pages were fetched randomly.
After inserted madvise(2), although 1-thread peformance has slightly decreased, it has scaled pretty better.