md: fix a fatal bug in check_and_move()

We can't rename(2) objects accross devices and this will cause recovery to fail.

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
md: make MD support as always enabled

Currently, we can plug new disks even MD isn't enabled. Following process

$ sheep /store # MD isn't enabled
$ collie node md plug /new_disk

will cause big trouble because plugging will succeed and force a recovery where
old /store isn't counted in the disk arrays.

To avoid such random careless opertaion and keep simplicity, this patch:

Make MD support as always enabled then the interface to be more upgradeing friendly:

   $ sheep /store # will enable MD and meta-store and object-store will share the same /store
   $ sheep /metastore,/disk1{,.../diskN,...} # works as before

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