io: add performance analysis of bio wrapper response.
kernel: add print_queue_limits().
bugfix: use blk_set_stacking_limits() for kernel 3.3 or later.
a bit refactor.
fix: print_queue_limits().
wlog: refactor.
wlog: a bit refactor.
bugfix: wlog: check fail without unit charactor.
tool: rename to walb_log.h.
wlog: use walb_log.h.
wlog: wlog-gen: support all-zero IO generation.
wlog: rename walb_util.hpp to walb_log.hpp.
remove wlog subdirectory.