staging/lustre/server: use unified request handler for MGS

- Unify request handler. It finds target for particular request and
  calls appropriate handler for it. Generic handlers are moved to
  the unified target code. The tgt_session_info is introduced to
  store all request-related data and passed to all handlers.
- Pack reply in llog server functions early and use err_serious()
- remove obsoleted llog_origin_handle_cancel(), it is not used
  anymore
- remove push_ctxt/pop_ctxt from llog server function, it is based
  on OSD now.

Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-2145
Lustre-change: http://review.whamcloud.com/4826


Signed-off-by: Mikhail Pershin <mike.pershin@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Signed-off-by: Peng Tao <bergwolf@gmail.com>
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>