why not osrm's file.index load with mmap function rather directly copy into memory

[hy05190134]

No description provided.

[danpat]

@hy05190134 You are welcome to make a pull request for that!

[hy05190134]

@danpat we find that if we just use mmap with .indexfile, it will make disk I/O when started with a lot of requests, so we use. preheat method to product page fault activity, later I will make a pull request for that

[hy05190134]

@danpat da70b98