LU-1866 lfsck: enhance otable-based iteration

1) Use ::load() method to unplug OSD layer iteration.

2) Use ::put() method to wakeup the LFSCK main engine
   if it is blocked at low layer for ::next() call.

Test-Parameters: envdefinitions=ENABLE_QUOTA=yes testlist=sanity-scrub
Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I28e5689ad3a4ee96a3f17a64e8d6dca553a86d77
Reviewed-on: http://review.whamcloud.com/4906
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Tested-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>