Refactor xlog writer

* Write xlog via fd instead of fiob with FILE * interface

* Remove O_DIRECT mode, use fdatasync(2)/sync_file_range(2) with
  posix_fadvise(2) to free the page cache. Disk cache polution
  on snapshot was tested with `vmtouch` utility and fadvise()
  provides similar result to O_DIRECT.

* Remove fiob.c implementation (unused)