LU-2145 server: use unified request handler for MGS

LU-2145 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.

Change-Id: Idded90d8112bcab74de9a58e7885fa17dd3340b8
Signed-off-by: Mikhail Pershin <mike.pershin@intel.com>
Reviewed-on: http://review.whamcloud.com/4826
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>