LU-1445 osp: rollover to the new seq synchronously

LU-1445 osp: rollover to the new seq synchronously

For simplicity, OSP will track only one sequence for precreation,
so it will need

1. Stop precreation when the current sequence is about to used up,
until all of the precreation objects has been used up.

2. ofd will precreate objects synchronously when the current sequence
is about to used up.

Signed-off-by: wang di <di.wang@intel.com>
Change-Id: I4c0de8f2869c9a8e7a4b0d2abe06041e84885b2e
Reviewed-on: http://review.whamcloud.com/4790
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>