b=20581 MDS returns full hash for readdir to decrease hash collision

b=20581 MDS returns full hash for readdir to decrease hash collision

1) MDS returns full hash (both major and minor, 64bit) for readdir to decrease
   hash collision.
2) Synchronization control between traversing thread and statahead thread when
   readdir.
Issues: LU-71
Change-Id: I043784d58607d474a501111c9690fdab89ce8a4a
Signed-off-by: nasf <yong.fan@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/281
Tested-by: Hudson
Reviewed-by: Cliff White <cliffw@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>