fsperf: Use sorted order by devs for fsperf-block.log

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
fsperf: Fix EMFILE error when there are many processes

If there are many processes, we open sched_*.dat for each process.  So,
this can be the cause of EMFILE.

To fix it, this patch uses "FileCache" module. It makes LRU cache of
open FHs, and when re-open expired cache, it opens by append mode.

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>