md: add dummy md proper

This add the dummy objects mapping code, which use consistent hashing, for md 
like support which aims to manage multiple disks in one node.

This patch just mainly add the compile support for md.c and some initilization
code modifcation to adopt to later series in the sheep core code.

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
util: add a helper to purge directory

This is preparation patch for md series.

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
sheep, farm: adapt to md support

This patch add another redirection inside mapping of oid-to-path, where we can 
play tricks that return obj_path in non-md mode and return actual path from 
underling md layer.

This also extends working directory to the sense that it is referred to all
the directories in the md array and add candy helpers to walk through
broadered WD.

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
sheep: teach stat_sheep to be md friendly

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
tests: teach tests to test md feature

To test md feature, just pass -md to check
 $ ./check -md ...

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
plain: refine init() interface

obj_path isn't a valid objects path for md mode, so we should not pass
obj_path to store.init(), then we'd better completely remove it since we don't
need it for now.

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
tests: add md group to group tests that is md capable

Usage:
 $ ./check -g md -md

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>