LU-3467 mdt: call MDT handlers via unified request handler

LU-3467 mdt: call MDT handlers via unified request handler

Move MDT to the unified request handler. All MDT-specific handlers
are swtched to use tgt_session_info instead of mdt_thread_info.
To minimize amount of changes the mdt_thread_info is initialized
just from tgt_session_info in most cases for now.

Signed-off-by: Mikhail Pershin <mike.pershin@intel.com>
Change-Id: I72e342cc900dc3dd34ca709542ad67de5015dd1f
Reviewed-on: http://review.whamcloud.com/6973
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>