raid: add dummy raid proper

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

This patch just mainly add the compile support for raid.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 raid series.

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

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

This also extends working directory to the sense that it is referred to all
the directories in the raid 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 raid friendly

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

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

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
tests/052: fix a spurious failure

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

obj_path isn't a valid objects path for raid 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: disable raid for tests that are not raid aware

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