add mb_at
delete cache->mb_array

	modified:   dm-lc.c
optimize. mb_at(cache, offset + i) -> seg->mb_array + i
optimize. remove segment_of
Merge branch 'topic/13_internal_mb' into v1-devel
change computation of refresh_segment
Merge branch 'topic/9_calc_refresh_segment' into v1-devel