LU-1445 fld: change md_site to seq_server_site

LU-1445 fld: change md_site to seq_server_site

Change md_site to seq_server_site, so both Metadata and data
stack will use this structure for sequence service.

Signed-off-by: wang di <di.wang@whamcloud.com>
Change-Id: I1f9c835d7006c4d336153c9bf8cac7d6fc729def
Reviewed-on: http://review.whamcloud.com/4805
Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>