LU-1406 ofd: add OBD methods to handle OST requests

LU-1406 ofd: add OBD methods to handle OST requests

Finally add OBD handlers to process incoming requests.
Add stub for quota functions, quota is non-functional in OFD.
Fix couple minor issues:
- slot checking in ofd_last_rcvd_update() must check lr_idx
- remove osd_write_locked() assertion from __osd_xattr_set(),
  it is too excessive there

Signed-off-by: Mikhail Pershin <tappro@whamcloud.com>
Change-Id: Iab270cdf5f2ad41df86845190c5c6db3436279c8
Reviewed-on: http://review.whamcloud.com/2894
Tested-by: Hudson
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>