LU-1866 lfsck: LFSCK main engine

LU-1866 lfsck: LFSCK main engine

There are two engines for LFSCK 1.5:

1) One is low layer object table based iteration, which can
   guarantee that all the objects can be scanned at least once.

2) The other is namespace based directory traversal. All the
   namespace based components, such as FID-in-dirent and linkEA,
   can be handled during that.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: Ic5a404cde35fdb20c24f6522faa8b21117cf4254
Reviewed-on: http://review.whamcloud.com/4909
Tested-by: Hudson
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>