LU-1187 ofd: Allocate ofd group dynamically.

Allocate the OFD sequence maps dynamically, so that the OST can
handle an arbitrary number of MDTs or WBC clients, and potentially
multiple streams of object allocations from each MDT.

Signed-off-by: Wang Di <di.wang@whamcloud.com>
Change-Id: Id5f8e5bf4f3dcbe3ba228144c7542bdd3cfef20c
Reviewed-on: http://review.whamcloud.com/4322
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Hudson
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>