LU-822 osd: multiple Object Index files

Single OI container could be performance bottleneck on server side,
because many service threads may content on the same OI htree-tree
even OI htree can support parallel operations but there are
still a lot of spinlock contentions and cacheline contentions.
Also, parallel operations of OI htree can't scale very well if
there are hundreds or thousands threads, it is because limitation
of dynlock. Instead of fix scalability of dynlock, the long term
solution is more straightforward, we can simply support multiple OI
containers and hash service threads to different OIs by lu_fid::f_seq.
We need to make sure this feature can support single OI created
by 2.1 or earlier versions, also, user can specify number of OIs by
modparameter osd_oi_num on creating new filesystem, this parameter
will be ignored if OSD is loading on existed filesystem.

Signed-off-by: Liang Zhen <liang@whamcloud.com>
Change-Id: Iaa5ef9e43b80301150608802e40b4ef506467457
Reviewed-on: http://review.whamcloud.com/1822
Tested-by: Hudson
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Fan Yong <yong.fan@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>